[MINOR] [MLLIB] [ML] [DOC] Minor doc fixes for StringIndexer and MetadataUtils

[jkbradley]

Changes:

• Make Scala doc for StringIndexerInverse clearer. Also remove Scala doc from transformSchema, so that the doc is inherited.
• MetadataUtils.scala: “ Helper utilities for tree-based algorithms” —> not just trees anymore

CC: @holdenk @mengxr

[holdenk]

I like the wording, much clearer :)

[SparkQA]

Test build #42234 has finished for PR 8679 at commit 316f60d.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

Merged into master. Thanks!

[jkbradley]

Thanks for checking! : )