Fix eldoc frame shrinking on some setups

[deifactor]

There appears to be an Emacs bug where in some environments, calling set-frame-size to another size and then to the original size will result in the second call not taking effect. This means that the workaround for #68 causes the frame to shrink if moving between doc items with the same doc size (or within the same doc item).

Instead, we just temporarily shrink the Markdown separators to make the window size calculations work, then restore them afterwards.

• eldoc-box.el (eldoc-box--update-childframe-geometry): Adjust markdown separator width instead of setting the frame to a tiny size.

[deifactor]

73345daa47a5612f298896e5eb044e10a79496e7 means we can close this.