[PATCH 9/9] doc: document user header indexing.

[David Bremner <david@tethera.net>]

It's a bit odd that the primary documentation is in notmuch-config,
but it is consistent with the "query:" prefix.
---
 doc/man1/notmuch-config.rst       | 9 +++++++++
 doc/man7/notmuch-search-terms.rst | 3 +++
 2 files changed, 12 insertions(+)

diff --git a/doc/man1/notmuch-config.rst b/doc/man1/notmuch-config.rst
index 89909808..28487079 100644
--- a/doc/man1/notmuch-config.rst
+++ b/doc/man1/notmuch-config.rst
@@ -195,6 +195,15 @@ The available configuration items are described below.
 
     Default: ``auto``.
 
+**index.header.<prefix>** **[STORED IN DATABASE]**
+    Define the query prefix <prefix>, based on a mail header. For
+    example ``index.header.List=List-Id`` will add a probabilistic
+    prefix ``List:`` that searches the ``List-Id`` field.  User
+    defined prefixes must not start with 'a'...'z'; in particular
+    adding a prefix with same name as a predefined prefix is not
+    supported. See **notmuch-search-terms(7)** for a list of existing
+    prefixes, and an explanation of probabilistic prefixes.
+
 **built_with.<name>**
     Compile time feature <name>. Current possibilities include
     "compact" (see **notmuch-compact(1)**) and "field_processor" (see
diff --git a/doc/man7/notmuch-search-terms.rst b/doc/man7/notmuch-search-terms.rst
index f7a39ceb..36ba804b 100644
--- a/doc/man7/notmuch-search-terms.rst
+++ b/doc/man7/notmuch-search-terms.rst
@@ -166,6 +166,9 @@ property:<key>=<value>
     can be present on a given message with several different values.
     See **notmuch-properties(7)** for more details.
 
+User defined prefixes are also supported, see **notmuch-config(1)** for
+details.
+
 Operators
 ---------
 
-- 
2.20.1