clipboard: add missing documentation for clipboard methods

[virus-rpi]

This adds the missing docs to the Clipboard class in the vlib/clipboard/x11/clipboard.c.v file.

• Added a new method check_availability to determine if the clipboard is available for use.
• Added a new method free to release the clipboard resources.
• Added a new method clear to clear the clipboard by setting it to an empty string.
• Added a new method has_ownership to check if the Clipboard has content ownership.
• Added a new method get_text to retrieve the current entry from the clipboard as a string.

[huly-for-github]

Connected to Huly®: V_0.6-22266

[spytheman]

Excellent work.

Thank you 🙇🏻‍♂️.