[PATCH 02 of 13 sparse] sparse: expand module docstring

[Gregory Szorc]

# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1498930563 25200
#      Sat Jul 01 10:36:03 2017 -0700
# Node ID 69e7300e65de0730c74db536fac0b047ef2c8ec1
# Parent  0c01dd19a7e2e57dd9d1abba3cb9643f37947c8c
sparse: expand module docstring

Clarify lack of BC guarantees. And say a bit more about the extension.

diff --git a/hgext/sparse.py b/hgext/sparse.py
--- a/hgext/sparse.py
+++ b/hgext/sparse.py
@@ -6,6 +6,17 @@
 # GNU General Public License version 2 or any later version.
 
 """allow sparse checkouts of the working directory (EXPERIMENTAL)
+
+(This extension is not yet protected by backwards compatibility
+guarantees. Any aspect may break in future releases until this
+notice is removed.)
+
+This extension allows the working directory to only consist of a
+subset of files for the revision. This allows specific files or
+directories to be explicitly included or excluded. Many repository
+operations have performance proportional to the number of files in
+the working directory. So only realizing a subset of files in the
+working directory can improve performance.
 """
 
 from __future__ import absolute_import