Issue #12306: Add ZLIB_RUNTIME_VERSION to the zlib module.

While we're at it, also document ZLIB_VERSION.

Patch by Torsten Landschoff.
diff --git a/Doc/library/zlib.rst b/Doc/library/zlib.rst
index 54835e7..aab9ec7 100644
--- a/Doc/library/zlib.rst
+++ b/Doc/library/zlib.rst
@@ -122,6 +122,7 @@
    won't fit into memory at once.  The *wbits* parameter controls the size of the
    window buffer.
 
+
 Compression objects support the following methods:
 
 
@@ -217,6 +218,26 @@
    seeks into the stream at a future point.
 
 
+Information about the version of the zlib library in use is available through
+the following constants:
+
+
+.. data:: ZLIB_VERSION
+
+   The version string of the zlib library that was used for building the module.
+   This may be different from the zlib library actually used at runtime, which
+   is available as :const:`ZLIB_RUNTIME_VERSION`.
+
+   .. versionadded:: 3.3
+
+
+.. data:: ZLIB_RUNTIME_VERSION
+
+   The version string of the zlib library actually loaded by the interpreter.
+
+   .. versionadded:: 3.3
+
+
 .. seealso::
 
    Module :mod:`gzip`