Update Eglot's manual about eglot-workspace-configuration

Update Eglot's manual about eglot-workspace-configuration

There is a possible use for a global setting
eglot-workspace-configuration, which is to (ab)use it to set
user-specific configuration when the server doesn't permit other
methods.  Rearrange the "Advanced server configuration" section and
describe that use in the manual.

* doc/misc/eglot.texi (Project-specific configuration): Correct
mistake about global usefulness of eglot-workspace-configuration.
(Advanced server configuration): Swap order of sections.
(User-specific configuration): Mention possibility of globally
setting eglot-workspace-configuration.