Document the new user.search_min_length setting

It provides a short introduction to and description of the setting, so
that users are aware that it's in place. This will also be documented in
the respective mobile app manuals, and in the config sample guide. This
fixes #4040.

user_manual/files/webgui/sharing.rst

@@ -32,6 +32,17 @@ folder with.
 If username auto-completion is enabled, when you start typing the user or group
 name ownCloud will automatically complete it for you, if possible.
 
+.. note::
+   From 10.0.8, user and group name search results are dependent on a new
+   `configuration setting`_, called ``user.search_min_length`` (it is set to 4 by
+   default).
+   This setting helps to aid search performance but requires that search
+   terms contain at least the defined number of characters.
+   Consequently, search terms shorter than the defined number of characters
+   will not return a list of matches.
+   However, they will return an existing user or group with a name of the same
+   length as the search term.
+
 After a file or folder has been shared, `Share Permissions`_ can be set on
 it. In the image below, you can see that the directory "event-Photos" is shared
 with the user "pierpont", who can *share*, *edit*, *create*, *change*, and
@@ -197,3 +208,4 @@ After the file's been successfully uploaded, they'll see (where possible) a prev
 .. Links
    
 .. _the Guest application: https://marketplace.owncloud.com/apps/guests
+.. _configuration setting: https://doc.owncloud.org/server/latest/admin_manual/configuration/server/config_sample_php_parameters.html