Replace "attribute" with "argument" to avoid confusion with PHP attributes

[GromNaN]

Q	A
Type	doc
BC Break	no
Fixed issues	Fix #2691

Summary

I prefer the word "parameter" as it's more a configuration than a method calls. For me "argument" refers to the definition in the method signature, and "parameter" to the value in the method call.

I kept the word "attribute" in the "xml-mappings" page as this is the correct word for XML tags.

Also improving examples and wording for "type", as the mapping from PHP property type to MongoDB Field type should be used by default for simplicity.
And a new paragraph about the date fields specifically, I think that was missing for newcomers that may make the mistake of storing dates as string.

[alcaeus]

Agree with the change from "attribute" to "argument" to reduce confusion. I'll let @jmikola take a look at the wording of the "Date Fields" section, but so far: LGTM.