[Dlang-internal] Documentation of makefiles

[Walter Bright via Dlang-internal]

Changes keep getting made and pulled for the makefiles with zero documentation. 
Makefiles are code and must be documented like any other piece of code.

Make targets:

1. should be documented as to whether they are public or private.

2. should have a description of what they are

Make variables:

1. should be documented as to whether they are public or private

2. should have a description of what they are

3. should document what their expected values are

Non-trivial sections, such as scripts, sed commands, etc., need to have a 
description of what they are doing and why.