Document hypertext escaping (#11374)

doc/lua_api.txt

@@ -2980,6 +2980,9 @@ Some tags can enclose text, they open with `<tagname>` and close with `</tagname
 Tags can have attributes, in that case, attributes are in the opening tag in
 form of a key/value separated with equal signs. Attribute values should not be quoted.
 
+If you want to insert a literal greater-than sign or a backslash into the text,
+you must escape it by preceding it with a backslash.
+
 These are the technically basic tags but see below for usual tags. Base tags are:
 
 `<style color=... font=... size=...>...</style>`