Merge updated manual pages from HEAD to RELENG_5.

1.21 src/lib/libipsec/ipsec_set_policy.3
1.14 src/lib/libipsec/ipsec_strerror.3
1.17,1.18 src/share/man/man4/ipsec.4

Clean up some descriptions and remove ambiguities in the language.
Add explanations to the examples.

Approved by: re (scottl)