Improve default ccache name API documentation

Document the lifetime and caching behavior of the
krb5_cc_default_name() return value.  Document that
krb5_cc_set_default_name() may be called with NULL to purge the cached
value.  Correct a typo in the krb5_cc_default() summary and explicitly
reference krb5_cc_default_name().

(back ported from commit 3e5fe754b9f9742d1c9b1564633d4172277166db)

https://github.com/krb5/krb5/commit/1467acb9392cb31c895586c186db5de19246328e
Author: Tom Yu <tlyu@mit.edu>
Commit: 1467acb9392cb31c895586c186db5de19246328e
Branch: krb5-1.11
 src/include/krb5/krb5.hin |   29 ++++++++++++++++++++++++-----
 1 files changed, 24 insertions(+), 5 deletions(-)