PEP 822: update d-string

[methane]

• Minimize difference from textwrap.dedent() by normalizing blank lines, including closing quote line.
• Clarify that the {} inside f/t-strings are used for indentation calculation but are not subject to dedent.
• Add document for new users and textwrap.dedent() users.

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)