Fix formatting and slightly update wxGTK install instructions

The only real change is that the part about building wxGTK under Cygwin
was removed, it wasn't very useful and shouldn't distract from other,
more important, information here.

docs/gtk/install.md

@@ -13,38 +13,31 @@ The simplest case                      {#gtk_simple}
 If you compile wxWidgets on Linux for the first time and don't like to read
 install instructions just do the following in wxWidgets directory:
 
-    > mkdir buildgtk
-    > cd buildgtk
-    > ../configure --with-gtk
-    > make
-    > sudo make install
-    > sudo ldconfig
+    $ mkdir buildgtk
+    $ cd buildgtk
+    $ ../configure --with-gtk
+    $ make
+    $ sudo make install
+    $ sudo ldconfig
 
-[if you get "ldconfig: command not found", try using "/sbin/ldconfig"]
+(if you get "ldconfig: command not found", try using `/sbin/ldconfig`)
 
-If you don't do the 'make install' part, you can still use the libraries from
-the buildgtk directory, but they may not be available to other users.
-
-If you want to remove wxWidgets on Unix you can do this:
-
-    > sudo make uninstall
-    > sudo ldconfig
+If you don't do the `make install` part, you can still use the libraries from
+the `buildgtk` directory, but they may not be available to other users.
 
 Note that by default, GTK 3 is used. GTK 2 can be specified
-with --with-gtk=2.
+with `--with-gtk=2` configure option.
 
-If you use CMake, please see @ref overview_cmake for
+If you want to use CMake, please see @ref overview_cmake for
 building wxWidgets using it.
 
 
 Troubleshooting                    {#gtk_errors_simple}
 ---------------
 
-IMPORTANT NOTE:
-
-  When sending bug reports tell us what version of wxWidgets you are
-  using (including the beta) and what compiler on what system. One
-  example: wxGTK 3.0.5, GCC 9.3.1, Fedora 31.
+*IMPORTANT NOTE:* When sending bug reports tell us the exact version of
+wxWidgets you are using as well as what compiler on what system. One example:
+wxGTK 3.0.5, GCC 9.3.1, Fedora 31.
 
 For any configure errors: please look at `config.log` file which was generated
 during configure run, it usually contains some useful information.
@@ -76,20 +69,6 @@ Now create your super-application myfoo.cpp and compile anywhere with
 
 
 
-Building wxGTK on Cygwin               {#gtk_cygwin}
---------------------------
-
-The normal build instructions should work fine on Cygwin. The one difference
-with Cygwin is that when using the "--enable-shared" configure option (which
-is the default) the API is exported explicitly using `__declspec(dllexport)`
-rather than all global symbols being available.
-
-This shouldn't make a difference using the library and should be a little
-more efficient. However if an export attribute has been missed somewhere you
-will see linking errors. If this happens then you can work around the
-problem by setting `LDFLAGS=-Wl,--export-all-symbols`. Please also let us know
-about it on the wx-dev mailing list.
-
 
 Configure options                        {#gtk_options}
 -----------------
@@ -257,7 +236,7 @@ Then you may install the library and its header files under
 have to perform the following command as root, using either `su`
 or `sudo`:
 
-        make install
+    $ make install
 
 After installing, you can run `make clean` in the original
 directory or just remove it entirely if you don't plan to build
@@ -283,17 +262,15 @@ in the following way (note that if you are copying and pasting
 this into your makefile, the leading spaces must be replaced by a
 `TAB` character):
 
-```make
-program: program.o
+    program: program.o
         $(CXX) -o program program.o `wx-config --libs`
 
-program.o: program.cpp
+    program.o: program.cpp
         $(CXX) `wx-config --cxxflags` -c program.cpp -o program.o
 
-clean:
+    clean:
         $(RM) program.o program
-.PHONY: clean
-```
+    .PHONY: clean
 
 If your application uses only some of wxWidgets libraries, you can
 specify required libraries when running wx-config. For example,