Agenda: Document the limitation of the filter preset

* lisp/org-agenda.el (org-agenda-filter-preset): Document the limitation
for the filter preset - it can only be used for an entire agenda view,
not in an individual block in a block agenda.
* doc/org.texi (Agenda commands):  Document the limitation
for the filter preset - it can only be used for an entire agenda view,
not in an individual block in a block agenda.

doc/org.texi

@@ -7603,7 +7603,9 @@ very fast, so that you can switch quickly between different filters without
 having to recreate the agenda@footnote{Custom commands can preset a filter by
 binding the variable @code{org-agenda-filter-preset} as an option.  This
 filter will then be applied to the view and persist as a basic filter through
-refreshes and more secondary filtering.}
+refreshes and more secondary filtering.  The filter is a global property of
+the entire agenda view - in a block agenda, you should only set this in the
+global options section, not in the section of an individual block.}
 
 You will be prompted for a tag selection letter, SPC will mean any tag at
 all.  Pressing @key{TAB} at that prompt will offer use completion to select a

lisp/org-agenda.el

@@ -2717,7 +2717,10 @@ removed from the entry content.  Currently only `planning' is allowed here."
 This must be a list of strings, each string must be a single tag preceded
 by \"+\" or \"-\".
 This variable should not be set directly, but agenda custom commands can
-bind it in the options section.")
+bind it in the options section.  The preset filter is a global property of
+the entire agenda view.  In a block agenda, it will not work reliably to
+define a filter for one of the individual blocks.  You need to set it in
+the global options and expect it to be applied to the entire view.")
 
 (defun org-prepare-agenda (&optional name)
   (setq org-todo-keywords-for-agenda nil)