Update Docs by revising --no_merge_train_val to --merge_train_val

[poppopting]

What does this PR do?

Update LibMultiLabel Documentation: revise flag --no_merge_train_val to flag --merge_train_val because the flag --no_merge_train_val is no longer used

Test CLI & API (bash tests/autotest.sh)

Test APIs used by main.py.

• [ ] Test Pass
  • (Copy and paste the last outputted line here.)
• [x] Not Applicable (i.e., the PR does not include API changes.)

Check API Document

If any new APIs are added, please check if the description of the APIs is added to API document.

• [ ] API document is updated (linear, nn)
• [x] Not Applicable (i.e., the PR does not include API changes.)

Test quickstart & API (bash tests/docs/test_changed_document.sh)

If any APIs in quickstarts or tutorials are modified, please run this test to check if the current examples can run correctly after the modified APIs are released.