doc-dyndbg: describe "class CLASS_NAME" query support

Add an explanation of the new "class CLASS_NAME" syntax and meaning,
noting that the module determines if CLASS_NAME applies to it.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/r/20220904214134.408619-19-jim.cromie@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/admin-guide/dynamic-debug-howto.rst

@@ -35,6 +35,7 @@ Dynamic debug has even more useful features:
    - line number (including ranges of line numbers)
    - module name
    - format string
+   - class name (as known/declared by each module)
 
  * Provides a debugfs control file: ``<debugfs>/dynamic_debug/control``
    which can be read to display the complete list of known debug
@@ -142,6 +143,7 @@ against.  Possible keywords are:::
 		 'file' string |
 		 'module' string |
 		 'format' string |
+		 'class' string |
 		 'line' line-range
 
   line-range ::= lineno |
@@ -203,6 +205,15 @@ format
 	format "nfsd: SETATTR"  // a neater way to match a format with whitespace
 	format 'nfsd: SETATTR'  // yet another way to match a format with whitespace
 
+class
+    The given class_name is validated against each module, which may
+    have declared a list of known class_names.  If the class_name is
+    found for a module, callsite & class matching and adjustment
+    proceeds.  Examples::
+
+	class DRM_UT_KMS	# a DRM.debug category
+	class JUNK		# silent non-match
+
 line
     The given line number or range of line numbers is compared
     against the line number of each ``pr_debug()`` callsite.  A single