[PATCH] Document the behaviour of predicate w.r.t. weak_ref

The behaviour of weak_ref attributes and predicate may be surprising to some;
document that in this instance, 'has_foo' does not return whether the object has
an attribute, just that it 'had' the attribute.
---
 lib/Moose.pm |    6 +++++-
 1 files changed, 5 insertions(+), 1 deletions(-)

diff --git a/lib/Moose.pm b/lib/Moose.pm
index 9912e2c..2ee9ba0 100644
--- a/lib/Moose.pm
+++ b/lib/Moose.pm
@@ -474,7 +474,8 @@ say anything about the attribute's value, which can be C<undef>.
 
 This will tell the class to store the value of this attribute as a weakened
 reference. If an attribute is a weakened reference, it B<cannot> also be
-coerced.
+coerced. Note that when weak refs expire, they are still considered 'set'
+in terms of predicate, default, etc.
 
 =item I<lazy =E<gt> (1|0)>
 
@@ -682,6 +683,9 @@ information.
 Creates a method to perform a basic test to see if a value has been set in the
 attribute. See the L<predicate option docs in
 Class::MOP::Attribute|Class::MOP::Attribute/predicate> for more information.
+This checks that the slot has been created (exists), so if your attribute is
+set to be a weak_ref then the predicate may return true when there is no actual
+value there.
 
 =item I<lazy_build> => (0|1)
 
-- 
1.7.0.4