doc: add BSON_INITIALIZER doc.
doc: add BSON_INITIALIZER doc.

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