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

[Aaron Ballman via cfe-commits]

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@@ -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
+  name, the preprocessor will be used to perform the inclusion or expand the
+  macro name into its replacement list, and return the resulting non-preprocessor
+  token.
+* Parser -- the part of the compiler responsible for determining syntactic
+  correctness of the source code. The parser will request tokens from the lexer
+  and after performing semantic analysis of the production, generates an
----------------
AaronBallman wrote:

Funny enough, I had written something for it but noticed we don't talk about Sema much in the user's manual (more of an internals manual thing). But I'll add it back and add a link to the internals manual here as well.

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