manywarnings: Overhaul documentation.

manywarnings: Overhaul documentation.

* doc/manywarnings.texi: In the example, put all unwanted warning
options into 'nw', and use a second gl_MANYWARN_COMPLEMENT invocation to
sort out how these options need to get added to WARN_FLAGS.
Describe the first-time use in more detail: Recommend a new GCC.
Recommend to test builds with -O2 and with -O0. Suggest to sort the
warning by warning option. Add reference to the GCC pragma's
documentation.