doc: clean up boolean vs. probabilistic prefixes

doc: clean up boolean vs. probabilistic prefixes

sphinx-build emits a minor warning:

[...]doc/man7/notmuch-search-terms.rst:223: WARNING: Block quote ends without a blank line; unexpected unindent.

And the tabular representation of boolean or probabilistic prefixes
currently renders like this when i view it in man:

       ┌───────────────────────────┬────────────────────────────┐
       │Boolean                    │ Probabilistic              │
       └───────────────────────────┴────────────────────────────┘

       │          tag: id:         │           from: to:        │
       │                           │                            │
       │       thread:     folder: │        subject:    attach‐ │
       │       path:               │        ment: mimetype:     │
       └───────────────────────────┴────────────────────────────┘

This isn't just ugly: it's confusing, because it seems to imply that
some of the prefixes in the left-hand column are somehow related to
specific other prefixes in the right-hand column.

The Definition List representation introduced by this patch should be
simpler for readers to understand, and doesn't have the warning.