doc: add visitor documentation.
doc: add visitor documentation.

rcsanchez97/libbson,ajdavis/libbson,alexeyvo/libbson,mongodbinc-interns/libbson,Convey-Compliance/libbson,ajdavis/libbson,bjori/libbson,Machyne/libbson,4Second2None/libbson_tmp,4Second2None/libbson_tmp,bjori/libbson,ksuarz/libbson,rayyang2000/libbson,paulmelnikow/libbson,paulmelnikow/libbson,mapr/libbson,paulmelnikow/libbson,mapr/libbson,mongodb/libbson,mongodb/libbson,Machyne/libbson,bjori/libbson,rcsanchez97/libbson,jqk6/libbson,remicollet/libbson,mapr/libbson,mongodbinc-interns/libbson,ajdavis/libbson,ksuarz/libbson,jqk6/libbson,rcsanchez97/libbson,paulmelnikow/libbson,remicollet/libbson,Machyne/libbson,rayyang2000/libbson,rayyang2000/libbson,mongodbinc-interns/libbson,jqk6/libbson,derickr/libbson,Convey-Compliance/libbson,4Second2None/libbson_tmp,alexeyvo/libbson,ksuarz/libbson,remicollet/libbson,mongodb/libbson,mongodbinc-interns/libbson,rayyang2000/libbson,derickr/libbson,Machyne/libbson,paulmelnikow/libbson,Convey-Compliance/libbson,4Second2None/libbson_tmp,alexeyvo/libbson,jqk6/libbson,remicollet/libbson,derickr/libbson,derickr/libbson