docs: update StringSliceVar flags that support repetition
Description
Improves the help documentation for command-line flags that accept multiple values. The changes add clarifying text to flag descriptions, specifically noting "Repeat flag for multiple [items]" to help users understand they can use the same flag multiple times to specify multiple values (like multiple labels, assignees, or file paths).
These are purely documentation improvements - no actual functionality changed, just better user guidance on how to properly use flags that accept multiple values.