Add support for WXWEBREQUEST_BACKEND environment variable

Defining this variable overrides the default backend choice.

Document the variable itself and also extend wxWebSession::New()
documentation.

docs/doxygen/overviews/envvars.h

@@ -26,5 +26,11 @@ wxWidgets programs.
         @c /usr/local or @c /usr). You can set WXPREFIX if you are for example
         distributing a binary version of an application and you don't know in advance
         where it will be installed.}
+@itemdef{WXWEBREQUEST_BACKEND,
+         This variable can be set to override the choice of the default backend
+         used by wxWebRequest, see wxWebSession::New(). Most common use is to
+         set it to @c "CURL" to force using libcurl-based implementation under
+         MSW or macOS platforms where the native implementation would be chosed
+         by default.}
 */