[tldoc] Portable installations

[Takuto ASAKURA]

Hi Karl, hi Norbert,

I have a small request to update the TeX Live Guide. Currently, the portable
installation feature of the TeX Live installer is described as follows:

> 4.2 Portable (USB) installations
> 
> The -portable installer option (or V command in the text installer or
> corresponding GUI option) creates a completely self-contained TeX Live
> installation under a common root and forgoes system integration. You can
> create such an installation directly on a USB stick, or copy it to a USB
> stick afterwards.

This instruction is good because it is brief and precise. However, I also
want to see how "a completely self-contained TeX Live installation" is
technically different from the normal installation.

Since various TEXMF trees are already introduced in Section 2.3, the readers
should be ready to learn with a bit more technical details. Thus, my proposal
is to add a description something like the following:

> Technically, with the portable installation, the default values of
> TEXMFHOME, TEXMFVAR, and TEXMFCONFIG will be set equivalent to TEXMFLOCAL,
> TEXMFSYSVAR, and TEXMFSYSCONFIG respectively, so that per-user configuration
> and caches will not be created.


Best,
Takuto