Main guide: clarify meaning of plain keyword query

Fixes #2463.

docs/guides/main.rst

@@ -238,7 +238,9 @@ songs.  Thus::
     $ beet ls album:bird
     The Mae Shi - Terrorbird - Revelation Six
 
-As you can see, search terms by default search all attributes of songs. (They're
+By default, a search term will match any of a handful of :ref:`common
+attributes <keywordquery>` of songs.
+(They're
 also implicitly joined by ANDs: a track must match *all* criteria in order to
 match the query.) To narrow a search term to a particular metadata field, just
 put the field before the term, separated by a : character. So ``album:bird``

docs/reference/query.rst

@@ -6,6 +6,8 @@ searches that select tracks and albums from your library. This page explains the
 query string syntax, which is meant to vaguely resemble the syntax used by Web
 search engines.
 
+.. _keywordquery:
+
 Keyword
 -------