path/filepath: clarify Dir and Clean documentation for Windows

[sapbotgit]

Fixes #77314

Updates documentation to clarify behavior on Windows:

• Dir("C:") returns "C:." not "C:" or "C:"
• Clean("C:") returns "C:." without trailing slash

The current documentation is misleading because it suggests that "C:" would return "C:" but it actually returns "C:."

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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