Fix docs for encode overload

Discussion: http://forum.dlang.org/post/m7nko1$udj$1@digitalmars.com

Fixed:
> 1. No 'Return:' block, though it obviously returns a value.
> 2. No 'Params:' block, though it obviously has parameters.
> 5. No description of what 'Tgt' is.
> 7. No clue where the variable 'c' comes from.
> 8. No clue where the type 'E' comes from.
> 9. 'R' is a type, not an instance. 

Not fixed (yet):
> 3. No 'Example:' block
> 4. No comparison with other 'encode' functions in the same module.

None of the `encode` functions have an example, and I'm no expert on encoding, so can't really help here.

> 6. No description of what 'Src' is.

I'm not sure whether to document this, because it perhaps should be inferred rather than explicitly passed.

Note: Github has renumbered my quoted items from Walter's post.