Clarify A_Clipboard as string in v1

[mikeyww]

From https://www.autohotkey.com/docs/v1/misc/Clipboard.htm:

[v1.1.35+]: A_Clipboard is an alias of Clipboard.

Add:

The clipboard variable(s) are incompatible with the binary number cache. Caching is disabled for "Clipboard" but not "A_Clipboard", so when assigning A_Clipboard, use a string instead of a binary number.

[lexikos]

It is a bug. However, I don't currently have any intention of supporting v1.