DOCS-277 additional clarification to getLog command reference

Author: Sam Kleinman

source/reference/commands.txt

@@ -1811,17 +1811,17 @@ Diagnostics
 
    Replace ``<log>`` with one of the following values:
 
-   - ``global`` - to generate the most recent log events from the
-     database. This is equivalent to running the ``tail`` command on
-     the :program:`mongod` log in the system shell.
+   - ``global`` - returns the combined output of all recent log
+     entries.
 
-   - ``rs`` - if the :program:`mongod` is part of a :term:`replica set`,
-     :dbcommand:`getLog` will generate the most recent log events
-     that relate to replication processes.
+   - ``rs`` - if the :program:`mongod` is part of a :term:`replica
+     set`, :dbcommand:`getLog` will return recent notices related to
+     replica set activity.
 
-   - ``startupWarnings`` - to generate logs that *may* contain
+   - ``startupWarnings`` - will return logs that *may* contain
      errors or warnings from MongoDB's log from when the current
-     process started.
+     process started. If :program:`mongod` started without warnings,
+     this filter may return an empty array.
 
    You may also specify an asterisk (e.g. ``*``) as the ``<log>``
    value to return a list of available log filters. The following
@@ -1834,8 +1834,8 @@ Diagnostics
       { "names" : [ "global", "rs", "startupWarnings" ], "ok" : 1 }
 
    :dbcommand:`getLog` returns events from a RAM cache of the
-   :program:`mongod` events, and *does not* read log data from the log
-   file.
+   :program:`mongod` events and *does not* read log data from the log
+   :file.
 
 .. dbcommand:: listDatabases