doc: Clarify that ffile-prefix-map applies to fcoverage-prefix-map, too [NFC] (llvm#117135)

Co-authored-by: MarcoFalke <*~=`'#}+{/-|&$^[email protected]>

Author: maflcko

clang/include/clang/Driver/Options.td

@@ -4405,7 +4405,7 @@ def fcoverage_prefix_map_EQ
     HelpText<"remap file source paths <old> to <new> in coverage mapping. If there are multiple options, prefix replacement is applied in reverse order starting from the last one">;
 def ffile_prefix_map_EQ
   : Joined<["-"], "ffile-prefix-map=">, Group<f_Group>,
-    HelpText<"remap file source paths in debug info, predefined preprocessor "
+    HelpText<"remap file source paths in debug info, coverage mapping, predefined preprocessor "
              "macros and __builtin_FILE(). Implies -ffile-reproducible.">;
 def fmacro_prefix_map_EQ
   : Joined<["-"], "fmacro-prefix-map=">, Group<f_Group>,