Doc fixes and improvements for syntax tables.

Doc fixes and improvements for syntax tables.

* src/syntax.c (Fmodify_syntax_entry): Doc fix.

* doc/lispref/syntax.texi (Syntax Basics): Rearrange the text for clarity.
Fix description of syntax table inheritance.
(Syntax Table Functions): Don't refer to internal contents of
syntax table, since that is not explained yet.  Copyedits.
(Standard Syntax Tables): Node deleted.
(Syntax Table Internals): Misc clarifications.  Improve table
formatting.

* doc/lispref/keymaps.texi (Inheritance and Keymaps):
* doc/lispref/text.texi (Sticky Properties): Tweak index entry.