doc: Examples of notmuch-reindex use and crypto policy

doc: Examples of notmuch-reindex use and crypto policy

Currently, notmuch has the levers needed to set coherent crypto policy
around how cleartext is indexed, which also has an impact on how
messages are rendered.  But we don't have a lot of documentation about
how to do sensible things.  This is an initial attempt to address
that.

The first example shows a way to selectively index specific messages.

The next two examples are about aligning the existing database with
crypto indexing policy

The default crypto policy is to not index cleartext, and to only
decrypt messages on display when explicitly requested.

The other sensible crypto policy is to index cleartext while stashing
session keys. messages indexed in this way will be searchable, and
will be decrypted on display automatically unless the user explicitly
asks for it to *not* be decrypted.

The policy for indexing *new* messages is stored in the database as
the config variable index.decrypt.

But setting policy for new messages doesn't retroactively affect
already indexed messages.

This patch attempts to document ways that someone can efficiently
align their pre-existing database with their new policy.

I'm not sure this is the right place to document these examples, but i
do want them to be user-facing and relatively easy to find.  I'm happy
to entertain suggestions for where else we should put them.