Fix small documentation problems

[zverok]

Fix part of the problems described in #19247:

• rendering of SyntaxError#path (and LoadError#path);
• Update Struct docs about keyword initialization
• Add list of keys to Time#deconstruct_keys fixed by @k-tsj b7bb14b
• Separate documentation for UnboundMethod#== from Method#== (behavior is different)
• Fix small glitch in GC docs (wrong rendering of mid-doc indent)
• path: option for IO.new
• Fix problems with Fiber's docs
  • References to Scheduler (was outdated to SchedulerInterface)
  • References between new methods (storage, [], []=)
  • storage call-sequence (rendered confusingly)

[nobu]

The tags need to be [DOC] to skip unnecessary CIs, instead of Docs:.