Cursor: update docs to clarify Cursor only works with in-memory buffers
Cursor: update docs to clarify Cursor only works with in-memory buffers

Reduce misconceptions about Cursor being more general than it really is.

Fixes: #52470

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust