[PATCH] man: add notes about issues with notmuch-count

[Sergei Shilovsky]

1. notmuch count --output=messages outputs estimated number

2. notmuch count --output=(files|threads) can be slow
---
 doc/man1/notmuch-count.rst | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/doc/man1/notmuch-count.rst b/doc/man1/notmuch-count.rst
index ca78c18..3bb6981 100644
--- a/doc/man1/notmuch-count.rst
+++ b/doc/man1/notmuch-count.rst
@@ -10,7 +10,7 @@ SYNOPSIS
 DESCRIPTION
 ===========
 
-Count messages matching the search terms.
+Count (an estimate of) the number of messages matching the search terms.
 
 The number of matching messages (or threads) is output to stdout.
 
@@ -25,16 +25,20 @@ Supported options for **count** include
     ``--output=(messages|threads|files)``
 
         **messages**
-            Output the number of matching messages. This is the default.
+            Output an estimate of the number of matching messages. This is the
+            default.
 
         **threads**
-            Output the number of matching threads.
+            Output the exact number of matching threads. The search is
+            performed, so this is a significantly heavier operation than
+            ``--output=messages``.
 
         **files**
-            Output the number of files associated with matching
+            Output the exact number of files associated with matching
             messages. This may be bigger than the number of matching
             messages due to duplicates (i.e. multiple files having the
-            same message-id).
+            same message-id). The search is performed, so this is a
+            significantly heavier operation than ``--output=messages``.
 
     ``--exclude=(true|false)``
         Specify whether to omit messages matching search.tag\_exclude
-- 
2.1.0