Fix docs for encode overload

[ntrel]

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.
3. No description of what 'Tgt' is.
4. No clue where the variable 'c' comes from.
5. No clue where the type 'E' comes from.
6. 'R' is a type, not an instance.

Not fixed (yet):

1. No 'Example:' block
2. 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.

1. 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.