docs: Add missing `namespace` parameter documentation [skip ci]
Although the `namespace` parameter was implemented in 0.37, its
documentation is still missing.
It has been added to the `gtkdoc`'s  documentation.
@@ -333,6 +333,7 @@ of the module.
* `main_xml`: specifies the main XML file
* `main_sgml`: equal to `main_xml`
* `mkdb_args`: a list of arguments to pass to `gtkdoc-mkdb`
+* `namespace`: specifies the name space to pass to `gtkdoc-mkdb`
* `module_version`: the version of the module, affects the installed location and the devhelp2 file location
* `scan_args`: a list of arguments to pass to `gtkdoc-scan`
* `scanobjs_args`: a list of arguments to pass to `gtkdoc-scangobj`
