Clarify point of setting dirname in load_standard_extensions

Changing dirname to alter how os.walk visits directories is a little
known feature, so add a note inline explaining how it works.

This came up because someone filed a bug saying this block of code was
confusing.

Closes-Bug: #1399546
Change-Id: I4cab323fcc7a4e2aab2ca7c36d6b9505b6a05bcb
Merge "Clarify point of setting dirname in load_standard_extensions"