Due to a typo, the defvar eglot--command-history wasn't actually used
in eglot-guess-contact as intended. That function used a
single-dash-name version of the variable instead.
This worked fine, except that two variables were created instead of
one, and the one actually being used didn't have any docstring.
Rename the variable to eglot-command-history to fix this. It's better
than renaming the reference in eglot-guess-contact which would lose
user's history for M-x eglot.
* lisp/progmodes/eglot.el (eglot-command-history): Rename from
eglot--command-history.