Rename wxHAVE_DPI_INDEPENDENT_PIXELS to wxHAS_XXX and document it

Using wxHAS_ prefix is more consistent with all the other similar
symbols, using wxHAVE_ was a mistake, that we have to pay for by
preserving the old name now (as it is actually already used in some code
outside of the library).

The fact that it's used also shows that it's better to document this
symbol, even if just to explain that it normally shouldn't be used, as
we can't really hide it anyhow.

docs/doxygen/mainpages/const_cpp.h

@@ -176,6 +176,13 @@ Currently the following symbols exist:
     supports template Read() and Write() methods in wxConfig.}
 @itemdef{wxHAS_DEPRECATED_ATTR, Defined if C++14 @c [[deprecated]] attribute is
     supported (this symbol only exists in wxWidgets 3.1.6 or later).}
+@itemdef{wxHAS_DPI_INDEPENDENT_PIXELS, Defined if pixel coordinates on the
+    current platform scale with DPI, i.e. if the given length in pixels has the
+    same apparent size on the display independently of the DPI (this symbol
+    only exists in wxWidgets 3.1.6 or later). Note that it should rarely, if
+    ever, be necessary to use this symbol directly, functions such as
+    wxWindow::FromDIP() and wxBitmap::GetScaledSize() exist to hide the
+    differences between the platforms with and without DPI-independent pixels.}
 @itemdef{wxHAS_MEMBER_DEFAULT, Defined if the currently used compiler supports
     C++11 @c =default.}
 @itemdef{wxHAS_LARGE_FILES, Defined if wxFile supports files more than 4GB in