doc: notmuch help search-terms, boolean operators

[Florian Friesdorf]

`notmuch help search-terms` currently states: "In addition to
individual terms, multiple terms can be combined with Boolean operators
("and", "or", "not", etc.).".

I would like to replace the "etc" with a full list and document the
operator precedence.

In lib/notmuch.h for notmuch_query_create I found a pointer to xapian
docs [2], which states that AND takes precedence over OR, but nothing
for NOT and XOR.

Through playing with `notmuch tag` and `notmuch search
--output=messages` I found:

Complete list of boolean operators in order of precedence:
- NOT
- AND
- XOR
- OR

Is this correct? If yes, I would extend the manpage accordingly.

[2] http://xapian.org/docs/queryparser.html

-- 
Florian Friesdorf <flo at chaoflow.net>
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