Document abbreviation style things (#4996)

We had a long discussion of this, so trying to document what seems to be
the conclusion... and also clean up the exceptions that I could find.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

testing/base/source_gen.cpp

@@ -309,7 +309,7 @@ static auto EstimateAvgClassDefLines(SourceGen::ClassParams params) -> double {
   return avg;
 }
 
-auto SourceGen::GenAPIFileDenseDecls(int target_lines,
+auto SourceGen::GenApiFileDenseDecls(int target_lines,
                                      const DenseDeclParams& params)
     -> std::string {
   RawStringOstream source;