[DOC] StringIO doc enhancements

[BurdetteLamar]

Treated:

• ::new
• ::open

• string

• string=

• close

• close_read

• close_write

• closed?

• closed_read?

• closed_write?

• eof?

[BurdetteLamar]

@nobu, I will merge this only after requesting your approval. Meanwhild, @peterzhu2118 and I can thrash out details?

[BurdetteLamar]

@nobu, I don't want to merge without your approval.

[BurdetteLamar]

@nobu, hoping for your approval. There's more I can do here once this is merged.

[BurdetteLamar]

@nobu, you're the maintainer here, right? Any further review?

[BurdetteLamar]

@nobu, let me know if you have further comments.