Replacements by DCGs. DOC additions.

* [DCG_CONTENT,SC] Renamed dcg_word//1 -> word//1.
* [DCG_CONTENT,SC] Removed dcg_word_atom//1.
* [DCG_GENERIC,NEW] Added dcg_call//[1,2,3,4,5].
* [DCG_GENERIC,NEW] Added dcg_switch//[2,3].
* [DCG_GENERIC,NEW] Added dcg_multi_list//3.
* [GV_ATTRS,DOC] Added documentation.
* [GV_DCG,DOC] Added documentation.
* [GV_DCG,SC] Simplified several DCG rules, using some of the new addition to module DCG_GENERIC.
* [ISO_639_1,ISO_639_2,ISO_639_3,ISO_639_5,RFC_3066,SC] Use of DCGs for ISO language and country tags.