DOC: Added ma.clump_masked to doc; removed `extras` in examples

1. Added `ma.clump_masked` and `ma.clump_unmasked` into the rst file;
2. Removed the `.extras` in the examples due to the namespace change
Merge pull request #5400 from yw5aj/master

DOC: Added ma.clump_masked to doc; removed `extras` in examples