[clang][doc] Provide documentation for FixedPointLiteral matcher (#125122)

Provide a documentation for FixedPointLiteral explaining which literals
will be matched and which are not. This documentation is compatible with
53e92e4
change.

Co-authored-by: Vladislav Aranov <vladislav.aranov@ericsson.com>

Author: earnol

clang/include/clang/ASTMatchers/ASTMatchers.h

@@ -2489,7 +2489,28 @@ extern const internal::VariadicDynCastAllOfMatcher<Stmt, FloatingLiteral>
 extern const internal::VariadicDynCastAllOfMatcher<Stmt, ImaginaryLiteral>
     imaginaryLiteral;
 
-/// Matches fixed point literals
+/// Matches fixed-point literals eg.
+/// 0.5r, 0.5hr, 0.5lr, 0.5uhr, 0.5ur, 0.5ulr
+/// 1.0k, 1.0hk, 1.0lk, 1.0uhk, 1.0uk, 1.0ulk
+/// Exponents 1.0e10k
+/// Hexadecimal numbers 0x0.2p2r
+///
+/// Does not match implicit conversions such as first two lines:
+/// \code
+///    short _Accum sa = 2;
+///    _Accum a = 12.5;
+///    _Accum b = 1.25hk;
+///    _Fract c = 0.25hr;
+///    _Fract v = 0.35uhr;
+///    _Accum g = 1.45uhk;
+///    _Accum decexp1 = 1.575e1k;
+/// \endcode
+/// \compile_args{-ffixed-point;-std=c99}
+///
+/// The matcher \matcher{fixedPointLiteral()} matches
+/// \match{1.25hk}, \match{0.25hr}, \match{0.35uhr},
+/// \match{1.45uhk}, \match{1.575e1k}, but does not
+/// match \nomatch{12.5} and \nomatch{2} from the code block.
 extern const internal::VariadicDynCastAllOfMatcher<Stmt, FixedPointLiteral>
     fixedPointLiteral;