Updates to Documentation chapter of Lisp manual.

Updates to Documentation chapter of Lisp manual.

* doc/lispref/help.texi (Documentation, Documentation Basics, Help Functions):
Minor clarifications.
(Accessing Documentation): Clarify what documentation-property is
for.  Add xref to Keys in Documentation.

* doc/lispref/macros.texi (Defining Macros):
* doc/lispref/modes.texi (Derived Modes): Say "documentation string" instead
of docstring.

* doc/lispref/tips.texi (Documentation Tips): Don't recommend using * in
docstrings.