Document the handler max_grab config param

config/job_conf.xml.sample_advanced

@@ -336,9 +336,9 @@
         <!-- Job handler processes - for a full discussion of job handlers, see the documentation at:
              https://docs.galaxyproject.org/en/latest/admin/scaling.html
 
-             The <handlers> container tag takes two optional attributes:
+             The <handlers> container tag takes three optional attributes:
 
-               <handlers assign_with="method" default="id_or_tag"/>
+               <handlers assign_with="method" max_grab="count" default="id_or_tag"/>
 
                - `assign_with` - How jobs should be assigned to handlers. The value can be a single method or a
                  comma-separated list that will be tried in order. The default depends on whether any handlers and a job
@@ -352,6 +352,11 @@
                    - `db-transaction-isolation` - Database Transaction Isolation
                    - `db-skip-locked` - Database SKIP LOCKED
 
+               - `max_grab` - Limit the number of jobs that a handler will self-assign per jobs-ready-to-run check loop
+                 iteration. This only applies for methods that cause handlers to self-assign multiple jobs at once
+                 (db-skip-locked, db-transaction-isolation) and the value is an integer > 0. Default is to grab as many
+                 jobs ready to run as possible.
+
                - `default` - An ID or tag of the handler(s) that should handle any jobs not assigned to a specific
                  handler (which is probably most of them). If unset, the default is any untagged handlers plus any
                  handlers in the `job-handlers` (no tag) pool.