build.7: Improve make delete-old example

[ziaee]

Summary

Be more explicit that deleting old files and libraries needs to be
really after you're rebooted a third time and you're really actually
sure your applications aren't using old libs, following irc discussion.

Also adjust an example slightly to fit on 80 character console.

MFC after: 3 days

[fuz]

share/man/man7/build.7

1115

Should be consistent, also check for additional occurrences.

[ziaee]

switch examples consistently to /client, thanks @fuz!

[jhb]

share/man/man7/build.7

1025

I would be tempted to leave make delete-old here and only defer make delete-old-libs to the new paragraph. In practice I always run make delete-old right after etcupdate -B. It's only make delete-old-libs that I defer until after updating packages.