Documentation/kbuild: major edit of modules.txt sections 1-4

Omit needless words and sentences; reorganize and tighten sentence structure;
swap sections 2.2 and 2.3 for a more logical flow; remove section 3, therefore
shifting 4->3; add to explanations; and add section on building multiple modules.
Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>