gparamspecs: Recommend use of most specific GParamSpec types

It’s quite common to see a g_param_spec_pointer() used for GObject or
boxed types which, while not incorrect, does make memory management
unsafe, since no copying or reference counting can be performed
automatically.

Similarly, people often use g_param_spec_boolean() when an enum would be
more appropriate, cf.
    http://blog.ometer.com/2011/01/20/boolean-parameters-are-wrong/
Using enums also means that the set of allowable values can be extended
in future if needed.

In the hope that people who write code like that read the documentation,
mention the more specific types in the documentation.

https://bugzilla.gnome.org/show_bug.cgi?id=741779