syntax-highlighting: improved docs on pop window configuratin

This commit improved both the layer documentation of pop window configuration
and docstring of relavant layer variables.

layers/+checkers/syntax-checking/README.org

@@ -15,7 +15,7 @@
   - [[#enable-support-for-traditional-error-navigation][Enable support for traditional error navigation]]
   - [[#error-indication][Error indication]]
   - [[#auto-hide-tooltips][Auto hide tooltips]]
-  - [[#window-position-and-size][Window position and size]]
+  - [[#error-list-pop-window-configuration][Error list pop window configuration]]
 - [[#key-bindings][Key bindings]]
 
 * Description
@@ -137,18 +137,27 @@ are kept open until the cursor is moved.
     '((syntax-checking :variables syntax-checking-auto-hide-tooltips 10)))
 #+END_SRC
 
-** Window position and size
-You can set the frame position within the emacs window by setting the variable
-=syntax-checking-window-position=. Available values are ='bottom=, ='left=, ='right=,
-and ='top=. You can also set the initial size by setting =syntax-checking-window-width=
-and =syntax-checking-window-height= for width and height respectively.
+** Error list pop window configuration
+By default, the =flycheck-error-list= is displayed in a pop window to the bottom
+of the frame, with 30% of the frame's height.
+
+You can customize where the pop window is shown by setting the variable
+=syntax-checking-window-position= to one of ='bottom=, ='top=, ='left=, and
+='right=.
+
+You can also set the initial window width and height by setting
+=syntax-checking-window-width= and =syntax-checking-window-height= respectively,
+where an integer value is the number of columns it takes while a float value is
+the relative size to the current frame.
+
+For example, to have the pop window appear on the right side of the frame with
+60 columns:
 
 #+BEGIN_SRC emacs-lisp
   (setq-default dotspacemacs-configuration-layers
     '((syntax-checking :variables
-                       syntax-checking-window-position 'bottom
-                       syntax-checking-window-width 80
-                       syntax-checking-window-height 25)))
+                       syntax-checking-window-position 'right
+                       syntax-checking-window-width 60
 #+END_SRC
 
 * Key bindings

layers/+checkers/syntax-checking/config.el

@@ -79,15 +79,23 @@ Note only one of BITMAP and MARGIN-STR is used, which is dictated by
 
 (spacemacs|defc syntax-checking-window-position 'bottom
   "Popup window position."
-  '(choice (const bottom) (const left) (const right) (const top)))
+  '(choice :tag "Position"
+           (const :tag "Bottom" bottom)
+           (const :tag "Top" top)
+           (const :tag "Left" left)
+           (const :tag "Right" right)))
 
 (spacemacs|defc syntax-checking-window-width 0.3
-  "Popup window width in characters (int) or as percentage (float)."
-  '(number))
+  "Popup window width in columns (int) or as percentage (float)."
+  '(choice :tag "Width"
+           (integer :tag "Width")
+           (float :tag "Width (%)")))
 
 (spacemacs|defc syntax-checking-window-height 0.3
-  "Popup window height in characters (int) or as percentage (float)."
-  '(number))
+  "Popup window height in columns (int) or as percentage (float)."
+  '(choice :tag "Width"
+           (integer :tag "Width")
+           (float :tag "Width (%)")))
 
 
 ;; internals