docs: document the filtering options in tinyproxy.conf(5)

docs/man5/tinyproxy.conf.txt.in

@@ -221,6 +221,37 @@ The possible keywords and their descriptions are as follows:
     string value will be used as the host name in the Via header.
     Otherwise, the server's host name will be used.
 
+*Filter*::
+
+    Tinyproxy supports filtering of web sites based on URLs or
+    domains. This option specifies the location of the file
+    containing the filter rules, one rule per line.
+
+*FilterURLs*::
+
+    If this boolean option is set to `Yes` or `On`, filtering is
+    performed for URLs rather than for domains. The default is to
+    filter based on domains.
+
+*FilterExtended*::
+
+    If this boolean option is set to `Yes`, then extended POSIX
+    regular expressions are used for matching the filter rules.
+    The default is to use basic POSIX regular expressions.
+
+*FilterCaseSensitive*::
+
+    If this boolean option is set to `Yes`, then the filter rules
+    are matched in a case sensitive manner. The default is to
+    match case-insensitively.
+
+*FilterDefaultDeny*::
+
+    The default filtering policy is to allow everything that is
+    not matched by a filtering rule. Setting `FilterDefaultDeny`
+    to `Yes` changes the policy do deny everything but the domains
+    or URLs matched by the filtering rules.
+
 
 BUGS
 ----