etc: Comment why we're binding xrange

Just mention there are differences between python versions

Author: alexcrichton

src/etc/gdb_rust_pretty_printing.py

@@ -13,6 +13,9 @@
 import sys
 import debugger_pretty_printers_common as rustpp
 
+# We want a version of `range` which doesn't allocate an intermediate list,
+# specifically it should use a lazy iterator. In Python 2 this was `xrange`, but
+# if we're running with Python 3 then we need to use `range` instead.
 if sys.version_info.major >= 3:
     xrange = range