Improve source docstrings in __init__ methods

[davidhassell]

The source parameter of all __init__ methods says something along the lines of "Initialize the parameters from those of source" which, at best, is a little vague. It would be good to provide a bit more detail on what this really means.

See NCAS-CMS/cf-python#492 for more context and more detail.

I propose adding the same boilerplate text to each class. E.g. for cfdm.Field:

            source: optional
                Initialise the `Field` instance from the *source*
                object. All other parameters, apart from *copy*, are
                ignored and their values are instead inferred from
                *source* using the `Field` API. Any parameters
                that can not be retrieved from *source* are assumed to
                have their default value.

                Note that if *source* is a `Field` instance then
                ``cfdm.Field(source=source)`` is equivalent
                to ``source.copy()``.