Rework computation of string given to eldoc (again)

Rework computation of string given to eldoc (again)

Co-authored-by: Andreii Kolomoiets <andreyk.mad@gmail.com>
Also do some refactoring to join similar logic in
eglot-doc-too-large-for-echo-area and eglot--truncate-string.

* eglot.el (eglot-doc-too-large-for-echo-area): Now returns the
number of lines available.
(eglot--truncate-string): New helper.
(eglot--first-line-of-doc, eglot--top-lines-of-doc): Remove.
(eglot--update-doc): Use new helpers.

* eglot-tests.el (hover-multiline-doc-locus): New test

GitHub-reference: close https://github.com/joaotavora/eglot/issues/459