explanations.html: New section, empty for now.

2001-11-27  Phil Edwards  <pme@gcc.gnu.org>

	* docs/html/explanations.html:  New section, empty for now.
	* docs/html/17_intro/howto.html:  Cleanup.  Move unrelated link...
	* docs/html/23_containers/howto.html:  ...to here.  Break up and
	rewrap threading discussion to emphasize warning.  Move malloc text...
	* docs/html/ext/howto.html:  ...to here.  New section.  Describe
	allocators and __USE_MALLOC effects.
	* docs/html/ext/sgiexts.html:  Mention their code.

From-SVN: r47391