Improve wording around PXE booting nodes

This commit includes some minor changes in
wording regarding PXE booting, specificically
reminding the reader to consider which network
the node will PXE boot from.

Change-Id: I198ca2e73ded7551ac1e62ba58bf52e41612a073
Closes-Bug: 1382259
Merge "Improve wording around PXE booting nodes"