[clang] [clang][analyzer doc] Add basic description of checker 'core.CallAndMessage' (NFC). (PR #177179)

[Donát Nagy via cfe-commits]

================
@@ -83,7 +83,48 @@ Ensure the shift operands are in proper range before shifting.
 
 core.CallAndMessage (C, C++, ObjC)
 """"""""""""""""""""""""""""""""""
- Check for logical errors for function calls and Objective-C message expressions (e.g., uninitialized arguments, null function pointers).
+Check for logical errors for function calls and Objective-C message expressions
+(e.g., uninitialized arguments, null function pointers).
+
+This checker is a collection of related checks that can be turned on or off by
+checker options.
+
+* **FunctionPointer** Check for null or undefined function pointer at function
+  call.
+* **CXXThisMethodCall** Check for null or undefined ``this`` pointer at method
+  call.
+* **CXXDeallocationArg** Check for null or undefined argument of
+  ``operator delete``.
+* **ArgInitializedness** Check for undefined pass-by-value function arguments.
+* **ArgPointeeInitializedness** Check for undefined pass-by-reference (pointer
+  to constant value or constant reference) function arguments. In special cases
+  non-constant arguments are checked. This happens for C library functions where
+  it is required to initialize (at least partially) a passed structure which
+  is used for both input and output (for example last argument of ``mktime`` or
+  ``mbrlen``).
+* **ParameterCount** Check for correct number of passed arguments to functions
+  or ObjC blocks. This will warn if the actual argument count is less than the
+  required count (by the declaration).
+* **NilReceiver** Check whether the receiver in a message expression is ``nil``.
+* **UndefReceiver** Check whether the receiver in a message expression is
+  undefined.
+
+For example to turn off the check for parameter count, set the checker option
+``ParameterCount`` to ``false``. By default all of these checks is enabled
+except **ArgPointeeInitializedness** because this check is more likely to
+produce false positives.
+
+**Additional options**
+
+* **ArgPointeeInitializednessComplete** Controls when to emit the warning at the
+  **ArgPointeeInitializedness** check. If set to ``true`` the warning will be
+  emitted if any member of the passed structure is not initialized. If set to
+  ``false`` the warning is emitted only if all members (the complete structure)
+  are not initialized. The default is ``false``. (Arguments of C library
+  functions which require initialization are always checked as if the option
+  would be ``false``.)
----------------
NagyDonat wrote:

```suggestion
This checker is a collection of related checks that are controlled by checker
options. The following checks are all enabled by default, but can be turned off
by setting their option to ``false``:
* **FunctionPointer** Check for null or undefined function pointer at function
  call.
* **CXXThisMethodCall** Check for null or undefined ``this`` pointer at method
  call.
* **CXXDeallocationArg** Check for null or undefined argument of
  ``operator delete``.
* **ArgInitializedness** Check for undefined pass-by-value function arguments.
* **ParameterCount** Check for correct number of passed arguments to functions
  or ObjC blocks. This will warn if the actual argument count is less than the
  required count (by the declaration).
* **NilReceiver** Check whether the receiver in a message expression is ``nil``.
* **UndefReceiver** Check whether the receiver in a message expression is
  undefined.

There are two additional options which are set to ``false`` by default (because
they are more likely to produce false positives) but can be set to ``true`` to
activate additional checks:
* **ArgPointeeInitializedness** Check for undefined pass-by-reference (pointer
  to constant value or constant reference) function arguments. In special cases
  non-constant arguments are checked. This happens for C library functions where
  it is required to initialize (at least partially) a passed structure which
  is used for both input and output (for example last argument of ``mktime`` or
  ``mbrlen``).
* **ArgPointeeInitializednessComplete** Controls when to emit the warning at the
  **ArgPointeeInitializedness** check. If this option is ``false`` (the
  default), a ``struct`` is considered to be "initialized" when at least one
  member is initialized. When this option is set to ``true``, structures are only
  accepted as initialized only when all members are initialized. (Arguments of
  C library functions which require initialization are always checked as if the
  option would be ``false``.)
```
In this suggestion I reorganized the list of options by separating the enabled-by-default and disabled-by-default options. (I understand why you separated the ArgPointeeInitializednessComplete option – which just influences another check – from the others that are standalone checks, but I think the order that I suggest has more advantages overall.)

https://github.com/llvm/llvm-project/pull/177179