Add a usage note to 'CanonicalNamespaces' hook docs

It seems natural to defer namespace configuration until the namespace is
actually registered, but in the case of namespaces that are registered in
CanonicalNamespaces hook, doing so will not work or cause unpredictable
results. This patch adds a note of warning to hooks.txt. See bug 45031 for
context.

Change-Id: I3f2d56dc7ded5e00974de0baf1a84188eade823c
Merge "Add a usage note to 'CanonicalNamespaces' hook docs"