Clarifying comment, docs accuracy.

Author: gpshead

Doc/library/lzma.rst

@@ -373,13 +373,13 @@ options. Valid filter IDs are as follows:
 
   * :const:`!FILTER_ARM64`
 
-    Only works if :data:`LZMA_VERSION` and :data:`LZMA_RUNTIME_VERSION` are 5.4.0 or later.
+    Only works if :data:`LZMA_RUNTIME_VERSION` is 5.4.0 or later.
 
     .. versionadded:: 3.13
 
   * :const:`!FILTER_RISCV`
 
-    Only works if :data:`LZMA_VERSION` and :data:`LZMA_RUNTIME_VERSION` are 5.6.0 or later.
+    Only works if :data:`LZMA_RUNTIME_VERSION` is 5.6.0 or later.
 
     .. versionadded:: 3.13
 

Modules/_lzmamodule.c

@@ -25,6 +25,12 @@
 #endif
 
 
+/*
+ * If the lzma.h we're building against is so old as not to define these, this
+ * provides their equivalent values so that the names remain defined in Python
+ * regardless.  lzma.LZMA_RUNTIME_VERSION is exposed to Python and is what
+ * people can use to decide if they can use them at runtime.
+ */
 #ifndef LZMA_FILTER_ARM64
 #define LZMA_FILTER_ARM64       LZMA_VLI_C(0x0A)
 #endif