conf: improve matching rules a bit more

Handle "null", null, !null, "!null", !"null" and "!\"null\""
matches, copy some docs from wireplumber about the rules and
add some more cases.

doc/dox/config/pipewire.conf.5.md

@@ -434,13 +434,105 @@ The general rules object follows the following pattern:
     }
 ]
 ```
+Match rules are an array of rules.
 
-The rules is an array of things to match and what actions to perform
-when a match is found.
+A rule is always a JSON object with two keys: matches and actions. The matches key is used to
+define the conditions that need to be met for the rule to be evaluated as true, and the actions
+key is used to define the actions that are performed when the rule is evaluated as true.
+
+The matches key is always a JSON array of objects, where each object defines a condition that needs
+to be met. Each condition is a list of key-value pairs, where the key is the name of the property
+that is being matched, and the value is the value that the property needs to have. Within a condition,
+all the key-value pairs are combined with a logical AND, and all the conditions in the matches
+array are combined with a logical OR.
+
+The actions key is always a JSON object, where each key-value pair defines an action that is
+performed when the rule is evaluated as true. The action name is specific to the rule and is
+defined by the rule’s documentation, but most frequently you will see the update-props action,
+which is used to update the properties of the matched object.
+
+In the matches array, it is also possible to use regular expressions to match property values.
+For example, to match all nodes with a name that starts with my_, you can use the following condition:
+
+```
+matches = [
+  {
+    node.name = "~my_.*"
+  }
+]
+```
+
+The ~ character signifies that the value is a regular expression. The exact syntax of the regular
+expressions is the POSIX extended regex syntax, as described in the regex (7) man page. 
+
+In addition to regular expressions, you may also use the ! character to negate a condition. For
+example, to match all nodes with a name that does not start with my_, you can use the following condition:
+
+```
+matches = [
+  {
+    node.name = "!~my_.*"
+  }
+]
+```
+
+The ! character can be used with or without a regular expression. For example, to match all
+nodes with a name that is not equal to my_node, you can use the following condition:
+
+```
+matches = [
+  {
+    node.name = "!my_node"
+  }
+]
+```
+
+The null value has a special meaning; it checks if the property is not available
+(or unset). To check if a property is not set:
+
+```
+matches = [
+  {
+    node.name = null
+  }
+]
+```
+
+To check the existence of a property, one can use the !null condition, for example:
+
+```
+matches = [
+  {
+    node.name = "!null"
+  }
+  {
+    node.name = !null            # simplified syntax
+  }
+]
+```
+To handle the "null" string, one needs to escape the string. For example, to check
+if a property has the string value "null", use:
+
+```
+matches = [
+  {
+    node.name = "null"
+  }
+]
+```
+To handle anything but the "null" string, use:
+
+```
+matches = [
+  {
+    node.name = "!\"null\""
+  }
+  {
+    node.name = !"null"      # simplified syntax
+  }
+]
+```
 
-The available actions and their values depend on the specific rule
-that is used. Usually it is possible to update some properties or set
-some quirks on the object.
 
 # CONTEXT PROPERTIES RULES  @IDX@ pipewire.conf