Format of some docs are incorrect for building the doc website.
* fix a bug that `gensrc` dir can not be built with -j.
* fix ut bug of CreateFunctionTest
This CL make bitmap_count, bitmap_union, and bitmap_union_count accept any expression whose return type is bitmap as input so that we can support flexible bitmap expression such as bitmap_count(bitmap_and(to_bitmap(1), to_bitmap(2))).
This CL also create separate documentation for each bitmap UDF to conform with other functions.
