doc: add docs for bson_reader_t.
doc: add docs for bson_reader_t.

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