Minor fixes to the docs.

Minor fixes to the docs.

 doc/emacs/buffers.texi (Uniquify): Clarify the default uniquification.
 doc/emacs/indent.texi (Tab Stops): Improve wording.
 doc/emacs/cmdargs.texi (General Variables): Improve docs of
 EMACSLOADPATH.  Index all the environment variables.
 (Misc Variables): Index all the environment variables.

 doc/lispref/text.texi (Registers): Document register-read-with-preview.
 doc/lispref/internals.texi (Building Emacs): Improve indexing.