Recommend use of 'key: value' syntax instead of content= and key=

Using key="{{ value }}" syntax with copy's content parameter and authorized_key's key parameter can cause problems when the value contains quotes or newlines. This is especially noticeable when a lookup('file', …) returns a key with quoted key options (issue #6294). There have also been multiple regressions in this area between 1.6.8, 1.6.9, and devel.

The use of "key: value" syntax makes all these problems go away, so this commit recommends this form in the examples for the respective modules. (I feel a more general warning and guidance about when to use key=val and when to use key: val is also needed, but I'll leave it to the maintainers to determine a place for it.)