Eglot: replace eglot-prefer-plaintext with eglot-documentation-renderer

The old boolean 'eglot-prefer-plaintext' is replaced by the more
expressive 'eglot-documentation-renderer', which can hold a major-mode
symbol, t (plain text), or nil (auto-detect each time).  By selecting a
renderer once at startup the repeated per-request lookups are avoided,
which helps with the slowness reported in bug#81150.

* lisp/progmodes/eglot.el (eglot-prefer-plaintext): Declare obsolete
alias to 'eglot-documentation-renderer'.
(eglot-documentation-renderer): New defcustom, reworked from from
eglot-prefer-plaintext.
(eglot--accepted-formats): Use new variable.
(eglot--format-markup): Use new variable.

* etc/EGLOT-NEWS: Announce change.

* doc/misc/eglot.texi (Customization Variables): Document
  eglot-documentation-renderer.

1 files changed, 12 insertions, 0 deletions

diff --git a/doc/misc/eglot.texi b/doc/misc/eglot.texi
index d501fe32d5d..c7c296c24ff 100644
--- a/doc/misc/eglot.texi
+++ b/doc/misc/eglot.texi
@@ -996,6 +996,18 @@ same language server.  That file is still outside your project
 will consider it to be part of the workspace.  The default is
 @code{nil}.
 
+@cindex markdown renderer
+@item eglot-documentation-renderer
+This variable controls how Eglot renders at-point documentation
+imported from the server (@pxref{Eglot Features}).  By default, the
+variable's value is set during startup to a markdown renderer if
+available, either @code{markdown-ts-view-mode} or
+@code{gfm-view-mode}.  These utilities visually enhance the
+documentation content through fontification and other formatting.  If
+you set it to @code{t}, plain text will be requested from the server
+and no rendering is attempted.  If the variable's value is @code{nil},
+Eglot will attempt to find a suitable renderer every time.
+
 @item eglot-mode-map
 This variable is the keymap for binding Eglot-related command.  It is in
 effect only as long as the buffer is managed by Eglot.  By default, it