Apply font to fixed-pitch face as well

and document this properly.
Fixes #16171

core/core-dotspacemacs.el

@@ -366,8 +366,11 @@ pressing `<leader> m`. Set it to `nil` to disable it."
                                             :size 10.0
                                             :weight normal
                                             :width normal)
-  "Default font, or prioritized list of fonts. This setting has no effect when
-running Emacs in terminal."
+  "Default font or prioritized list of fonts. This setting has no effect when
+running Emacs in terminal. The font set here will be used for default and
+fixed-pitch faces. The `:size' can be specified as
+a non-negative integer (pixel size), or a floating-point (point size).
+Point size is recommended, because it's device independent. (default 10.0)"
   '(choice (cons string sexp)
            (repeat (cons string sexp)))
   'spacemacs-dotspacemacs-init)

core/core-fonts-support.el

@@ -55,11 +55,15 @@ The return value is nil if no font was found, truthy otherwise."
           ;; that the font is applied to future frames is to modify
           ;; default-frame-alist, and Customization causes issues, see
           ;; https://github.com/syl20bnr/spacemacs/issues/5353.
-          ;; INHIBIT-CUSTOMIZE is only present in recent emacs versions. 
+          ;; INHIBIT-CUSTOMIZE is only present in recent emacs versions.
           (if (version< emacs-version "28.0.90")
               (set-frame-font fontspec nil t)
             (set-frame-font fontspec nil t t))
           (push `(font . ,(frame-parameter nil 'font)) default-frame-alist)
+
+          ;; Make sure that our font is used for fixed-pitch face as well
+          (set-face-attribute 'fixed-pitch nil :family 'unspecified)
+
           ;; fallback font for unicode characters used in spacemacs
           (pcase system-type
             (`gnu/linux

core/templates/.spacemacs.template

@@ -253,7 +253,9 @@ It should only modify the values of Spacemacs settings."
    ;; (default t)
    dotspacemacs-colorize-cursor-according-to-state t
 
-   ;; Default font or prioritized list of fonts. The `:size' can be specified as
+   ;; Default font or prioritized list of fonts. This setting has no effect when
+   ;; running Emacs in terminal. The font set here will be used for default and
+   ;; fixed-pitch faces. The `:size' can be specified as
    ;; a non-negative integer (pixel size), or a floating-point (point size).
    ;; Point size is recommended, because it's device independent. (default 10.0)
    dotspacemacs-default-font '("Source Code Pro"

doc/DOCUMENTATION.org

@@ -1226,7 +1226,8 @@ The default font used by Spacemacs is [[https://github.com/adobe-fonts/source-co
 recommended to install it on your system if you wish to use it.
 
 To change the default font set the variable =dotspacemacs-default-font= in your
-=.spacemacs= file. By default its value is:
+=.spacemacs= file, this setting will be used for the =default= and =fixed-pitch= faces.
+By default its value is:
 
 #+BEGIN_SRC emacs-lisp
   (setq-default dotspacemacs-default-font '("Source Code Pro"