Docs: Warn about display_as
Docs: Warn about display_as

This commit warns about the potential issues (segfaults with
krb5 < 1.13.3 and incorrect behavior with canonicalized names)
in the inline docs for `display_as`.

Fixes #79

frozencemetery/python-gssapi,frozencemetery/python-gssapi,pythongssapi/python-gssapi,pythongssapi/python-gssapi,frozencemetery/python-gssapi,pythongssapi/python-gssapi