Help: Format and clarify source_group() documentation (#14631)

Refine the .rst formatting.  Document the FILES option explicitly
and specify how it treats relative paths.
Merge topic 'doc-source_group-FILES' into next

0060e95 Help: Format and clarify source_group() documentation (#14631)
Merge topic 'doc-source_group-FILES'

0060e95 Help: Format and clarify source_group() documentation (#14631)
Merge branch 'master' into next