docs(modeline): mention modeline hiding & strange icons

Ref: #1166

modules/ui/modeline/README.org

@@ -41,7 +41,14 @@ powered by the [[doom-package:][doom-modeline]] package (where you can find scre
  🔨 /This module's usage documentation is incomplete./ [[doom-contrib-module:][Complete it?]]
 #+end_quote
 
-** TODO Hiding the modeline
+** Hiding the modeline
+- You can use ~M-x hide-mode-line-mode RET~ to hide modeline for the current
+  buffer.
+- Also, there is ~global-hide-mode-line-mode~ for globally hiding modeline. You
+  can just add =(global-hide-mode-line-mode)= into =~/.doom.d/config.el~.
+- ~hide-mode-line-excluded-modes~ can be set for preventing
+  ~global-hide-mode-line-mode~ hiding modeline for buffers with the
+  corresponding major modes.
 
 ** TODO Switching the modeline and header line
 
@@ -67,9 +74,26 @@ powered by the [[doom-package:][doom-modeline]] package (where you can find scre
 I rarely need to know what minor modes are active, so I removed them. ~M-x
 doom/describe-active-minor-mode~ was written to substitute for it.
 
-** TODO Icons in my modeline look strange
+** Icons in my modeline look strange
+1. Check whether ~all-the-icons~ are installed. Run ~M-x
+   all-the-icons-install-fonts~ to install the resource fonts. Note that
+   ~all-the-icons~ only support GUI. See [[https://github.com/domtronn/all-the-icons.el][all-the-icons]] for details.
 
-** TODO The right side of the modeline is cut off
+2. ~cnfonts~ will conflict with ~all-the-icons~. You can refer the following
+   workaround:
+    #+begin_src emacs-lisp
+    ;; See https://github.com/seagle0128/doom-modeline/issues/278#issuecomment-569510336
+    ;; Add to $DOOMDIR/packages.el
+    (package! cnfonts)
+    ;; Add to $DOOMDIR/config.el
+    (add-hook 'after-setting-font-hook #'cnfonts-set-font)
+    #+end_src
+
+3. If ~all-the-icons~ fonts are installed while the icons cannot display
+   correctly, please install the non-free font [[https://dn-works.com/wp-content/uploads/2020/UFAS-Fonts/Symbola.zip][Symbola]]. This issue usually
+   occurs on Windows.
+
+** The right side of the modeline is cut off
 I believe the consensus is: this is due to oversized icons, i.e. a font issue.
 Some possible solutions: