[PSR-7] Remove `$maxLength` from StreamInterface::getContents()

- Per @mtdowling, this will be rarely if ever be used, and actually makes
  implementations overly complex.
Remove obsolete verbiage from docblock

- per @renan
Merge branch 'feature/stream-get-contents-maxlength-removal' of github.com:weierophinney/fig-standards into weierophinney-feature/stream-get-contents-maxlength-removal

Conflicts:
	proposed/http-message.md
Merge branch 'weierophinney-feature/stream-get-contents-maxlength-removal'