kconfig: sort found symbols by relevance

When searching for symbols, return the symbols sorted by relevance.

Sorting is done as thus:
  - first, symbols that match exactly
  - then, alphabetical sort

Since the search can be a regexp, it is possible that more than one symbol
matches exactly. In this case, we can't decide which to sort first, so we
fallback to alphabeticall sort.

Explain this (new!) sorting heuristic in the documentation.

Reported-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Jean Delvare <jdelvare@suse.de>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Roland Eggner <edvx1@systemanalysen.net>
Cc: Wang YanQing <udknight@gmail.com>

--
Changes v1->v2:
  - drop the previous, complex heuristic in favour of a simpler heuristic
    that is both easier to understand, *and* to maintain (Jean)
  - explain sorting heuristic in the doc  (Jean)