notmuch_thread_get_authors: document match grouping with |

as stated in thread.cc:115

/* Construct an authors string from matched_authors_array and
 * authors_array. The string contains matched authors first, then
 * non-matched authors (with the two groups separated by '|'). Within
 * each group, authors are listed in date order. */

this is, however, not reflected in the public API documentation in
notmuch.h:970. This patch a paragraph explaining how | separates the
group of authors of messages matching the query and those of messages
that do not, but are still contained in the thread.
test: simplify T360-symbol-hiding, use nm instead of objdump

After yet another variation in objdump output caused this test to fail
(on a Debian port, no less), I decided whatever putative benefit we
get from looking at the object files instead of the library isn't
worth the maintenence headache.

This version uses nm -P. nm -P should be portable, and fixed format.
It purposely doesn't use the -D argument, since that is non-POSIX and
nm on GNU/Linux seems do the right thing without it.

It still won't work out of the box on e.g. Mac OS/X. I think the right
thing to do there is to move some more configuration information into
sh.config.