[clang] [Docs] Some updates to the Clang user's manual (PR #151702)

[Erich Keane via cfe-commits]

================
@@ -60,29 +60,46 @@ features that depend on what CPU architecture or operating system is
 being compiled for. Please see the :ref:`Target-Specific Features and
 Limitations <target_features>` section for more details.
 
-The rest of the introduction introduces some basic :ref:`compiler
-terminology <terminology>` that is used throughout this manual and
-contains a basic :ref:`introduction to using Clang <basicusage>` as a
-command line compiler.
-
 .. _terminology:
 
 Terminology
 -----------
+* Lexer -- the part of the compiler responsible for converting source code into
+  abstract representations called tokens.
+* Preprocessor -- the part of the compiler responsible for in-place textual
+  replacement of source constructs. When the lexer is required to produce a
+  token, it will run the preprocessor while determining which token to produce.
+  In other words, when the lexer encounters something like `#include` or a macro
----------------
erichkeane wrote:

Ah-good to know!  I just know I missed `macro name` my first read through and was about to suggest "hey, maybe you should point out more than `#include`!" before I re-read it, so it didn't stand out.  perhaps it is better without word wrap.

https://github.com/llvm/llvm-project/pull/151702