Add shell completion section to PHB

[adamw]

Summary

• Always install shell completion files
• Don't register dependencies

I've not edited docproj stuff before, so I could really use some eyes here.

[mat]

en_US.ISO8859-1/books/porters-handbook/special/chapter.xml

5538–5540

OPTION is not a variable, so, lowercase it and remove the tag.

Also, never address the porter directly with "you", and keep sentences short, so, maybe something like:

It is not necessary to add an option for it.
If an option is added though, always enable it
in <varname>OPTIONS_DEFAULT</varname>.

Or some variation on it, your english is better than mine :-)

5549

Maybe remove the / at the end of the path, it is already a path, no need to add the extra delimiter at the end. (Same for the zsh path.)

5560–5561

extra new line after the end of the sentence.