"parts" is kind of vague. "source files" is more specific.

Needs a comma after "build".

The emphasis should be on just "really", I'd say:

No port really needs to be root.
No port really needs to be root.

What does "fake those calls" mean? If it is described elsewhere, a link on those three words would help.

The second sentence needs a pause after the second time. But it's better to have sentences that don't make the reader pause:
Slight patching of the port's Makefile is needed in some cases.

This should be followed by a sentence that tells what kind of patching would be done.

"For example, calls to blahblah can be patched to use zzyx."

End the sentence here, and the next line starts a new one.

s%</link>%</link>.%

s/it/It/

Serial comma needed: s/build/build,/

Mmm, it means, that if you run, say, chown bin foobar, it will mock up the chown call so that it returns "all ok" but without doing them for real, and failing because it's not running as root, say:

[mat@aragorn /usr/home/mat]$ id
uid=1000(mat) gid=1000(mat) groups=1000(mat),0(wheel),1010(poudriere)
[mat@aragorn /usr/home/mat]$ fakeroot
[mat@aragorn /usr/home/mat]$ id
uid=0(root) gid=0(wheel) groups=1000(mat),0(wheel),1010(poudriere)
[mat@aragorn /usr/home/mat]$ touch foobar
[mat@aragorn /usr/home/mat]$ ls -l foobar
-rw-r--r--  1 root  wheel  0 30 jui 15:06 foobar
[mat@aragorn /usr/home/mat]$ chown bin foobar
[mat@aragorn /usr/home/mat]$ ls -l foobar
-rw-r--r--  1 bin  wheel  0 30 jui 15:06 foobar
[mat@aragorn /usr/home/mat]$ exit
exit
[mat@aragorn /usr/home/mat]$ ls -l foobar
-rw-r--r--  1 mat  wheel  0 30 jui 15:06 foobar
[mat@aragorn /usr/home/mat]$ exit

It's a tool created by Debian to help building packages as a regular user, it just fakes being root, like its name suggests :-)

It's a bit hard to give example of the kind of patching that will need to be done, because most of it should be faked by USES=fakeroot, so, it's kinda left to the porter to figure out what is wrong with the software he is trying to make work.

In that case, we should tell the reader where they can find information on how this works. Maybe

See <link href="https://wiki.debian.org/FakeRoot"/> for more information on the <command>fakeroot</command> tool.

Mmmm, ok, though, I'll use http://fakeroot.alioth.debian.org/ which seems to be the project's homepage. And maybe add that on the USES page and not here.

Nice!