Document the macro and its usage

docs/doxygen/mainpages/const_cpp.h

@@ -375,6 +375,11 @@ more details.
         set to 1 for compatibility reasons as @c -DwxNO_UNSAFE_WXSTRING_CONV
         can be used only compiling the application code, without rebuilding the
         library. Support for this option appeared in wxWidgets 3.1.1.}
+@itemdef{wxNO_IMPLICIT_WXSTRING_ENCODING,
+        this symbol is not defined by wxWidgets itself, but can be defined by
+        the applications using the library to disable implicit
+        conversions from and to <tt>const char*</tt> in wxString class.
+        Support for this option appeared in wxWidgets 3.1.4.}
 @itemdef{WXMAKINGDLL_XXX,
         used internally and defined when building the
         library @c XXX as a DLL; when a monolithic wxWidgets build is used only a

docs/doxygen/overviews/string.h

@@ -242,6 +242,28 @@ and for the return value of c_str().
 For this conversion, the @a wxConvLibc class instance is used.
 See wxCSConv and wxMBConv.
 
+It is also possible to disable any automatic conversions from C
+strings to Unicode. This can be useful when the @a wxConvLibc encoding
+is not appropriate for the current software and platform. The macro @c
+wxNO_IMPLICIT_WXSTRING_ENCODING disables all implicit conversions, and
+forces the code to explicitly indicate the encoding of all C strings.
+
+@code
+wxString s;
+// s = "world"; does not compile with wxNO_IMPLICIT_WXSTRING_ENCODING
+s = wxString::FromAscii("world"); // Always compiles
+s = wxASCII_STR("world"); // shorthand for the above
+s = wxString::FromUTF8("world"); // Always compiles
+s = wxString("world", wxConvLibc); // Always compiles, explicit encoding
+
+const char *c;
+// c = s.c_str();  does not compile with wxNO_IMPLICIT_WXSTRING_ENCODING
+// c = s.mb_str(); does not compile with wxNO_IMPLICIT_WXSTRING_ENCODING
+c = s.ToAscii(); // Always compiles
+c = s.ToUTF8(); // Always compiles
+c = s.utf8_str(); // Alias for the above
+c = s.mb_str(wxConvLibc); // Always compiles, explicit encoding
+@endcode
 
 @subsection overview_string_iterating Iterating wxString Characters