Clarify use of contractions in diagnostic messages

This dissuades contributors from using contractions when writing
diagnostic wording for Clang. Contractions should be avoided because of
the potential for visual confusion with single quoting syntactic
constructs and because they can be harder to understand for non-native
English speakers.

Author: AaronBallman

clang/docs/InternalsManual.rst

@@ -160,6 +160,10 @@ wording a diagnostic.
   named in a diagnostic message. e.g., prefer wording like ``'this' pointer
   cannot be null in well-defined C++ code`` over wording like ``this pointer
   cannot be null in well-defined C++ code``.
+* Prefer diagnostic wording without contractions whenever possible. The single
+  quote in a contraction can be visually distracting due to its use with
+  syntactic constructs and contractions can be harder to understand for non-
+  native English speakers.
 
 The Format String
 ^^^^^^^^^^^^^^^^^