This adds support for changing the schema of a conversion, operator,
operator class, operator family, text search configuration, text search
dictionary, text search parser, or text search template.

Dimitri Fontaine, with assorted corrections and other kibitzing.

The endterm attribute is mainly useful when the toolchain does not support
automatic link target text generation for a particular situation.  In  the
past, this was required by the man page tools for all reference page links,
but that is no longer the case, and it now actually gets in the way of
proper automatic link text generation.  The only remaining use cases are
currently xrefs to refsects.

another section if required by the platform (instead of the old way of
building them in section "l" and always transforming them to the
platform-specific section).

This speeds up the installation on common platforms, and it avoids some
funny business with the man page tools and build process.

Notably, standardize on using "token" for the strings output by a parser,
while "lexeme" is reserved for the normalized strings produced by a
dictionary.

init options of the template as top-level options in the syntax.  This also
makes ALTER a bit easier to use, since options can be replaced individually.
I also made these statements verify that the tmplinit method will accept
the new settings before they get stored; in the original coding you didn't
find out about mistakes until the dictionary got invoked.

Under the hood, init methods now get options as a List of DefElem instead
of a raw text string --- that lets tsearch use existing options-pushing code
instead of duplicating functionality.

pages for the new SQL commands.  I also committed Bruce's text search
introductory chapter, as-is except for fixing some markup errors,
so that there would be a place for the reference pages to link to.