[Docs] Refine the docstring of EvalHook

[Yan-Daojiang]

Refine the EvalHook comments

Motivation

I noticed that according to the implementation of EvalHook, its parameter start can also work in iteration mode. However, only the epoch mode was stated in the original comment, so I improved the comment.

Modification

Improve the comments for EvalHook.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.