doc: Improve documentation strings in parse-tree directory. * lex.ll (F__display_tokens__): Add seealso reference. * lex.ll (F__token_count__): Add seealso reference. * lex.ll (F__lexer_debug_flag__): Document function. * oct-parse.in.yy (Fautoload): Add additional calling form. Rephrase several sentences. * oct-parse.in.yy (Fmfilename): Make single sentence description stand apart from the rest of documentation. * oct-parse.in.yy (Fsource): Make single sentence description stand apart from the rest of documentation. Add seealso link to 'run'. * oct-parse.in.yy (Fbuiltin): Change type to "Built-in Function" from "Loadable Function". * oct-parse.in.yy (Feval): Rephrase several sentences. Add programming note suggesting the use of alternatives like try/catch or unwind_protect. * oct-parse.in.yy (F__parser_debug_flag__): Document function. * pt-mat.cc (Fstring_fill_char): Use semicolon in place of period for stronger idea linkage.

--- a/libinterp/parse-tree/pt-mat.cc
+++ b/libinterp/parse-tree/pt-mat.cc
@@ -1374,7 +1374,7 @@
 @deftypefnx {Built-in Function} {@var{old_val} =} string_fill_char (@var{new_val})\n\
 @deftypefnx {Built-in Function} {} string_fill_char (@var{new_val}, \"local\")\n\
 Query or set the internal variable used to pad all rows of a character\n\
-matrix to the same length.  It must be a single character.  The default\n\
+matrix to the same length; It must be a single character.  The default\n\
 value is @qcode{\" \"} (a single space).  For example:\n\
 \n\
 @example\n\