string: add function docs.
string: add function docs.

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