doc: field processor support now always included, adjust manual pages

doc: field processor support now always included, adjust manual pages

The features that require field processor support, are now just
documented w/o mentioning **Xapian Field Processors**' is needed
for those.

Replaced "compact" and "field_processor" with "retry_lock" in
build_with config option, as it is currently the only one that
is optionally excluded. The former 2 are now documented as
features always included.

Dropped one 'we' "passive" in notmuch-search-terms.rst. It was the
only one, and inconsistent with rest of the documentation in that
file.

Dropped message about conditional open-ended ranges support, as
those are now always supported.