Document indent behaviour for directives

[latosha-maltba]

Subject: Document indent behaviour for directives

Mention how the indent of the contents for directives is chosen and what
the effect on directives which are sensitive to whitespace is,
e.g. code-block.

Feature or Bugfix

• Refactoring?

Purpose

• Spin-off of code-block: Fix handling of :dedent: and add unit tests #9661 (comment)
• This caveat of whitespace parsing threw me of back then when learning Sphinx/ReST, wished is was documented more prominently somewhere. Now I propose this addition to the docs to close this gap.

[tk0miya]

LGTM. Merging.