Explain use of EESSI_ACCELERATOR_INSTALL better. #96

Today the documentation for EESSI_ACCELERATOR_INSTALL says:

In these cases, if you wish to create an accelerator installation you must set the environment variable EESSI_ACCELERATOR_INSTALL (and load/reload this module).

"if you wish to create a accelerator installation" can be read as "if you are looking to create a new installation base using a previously unknown accelerator... Ie, something other than CUDA.

This can lead users (hi!) to not set the variable, and then see the following when building a module that is supposed to use an accelerator:

== FAILED: Installation ended unsuccessfully: It seems you are trying to install an accelerator package magma into a non-accelerator location /cvmfs/software.eessi.io/host_injections/2023.06/software/linux/x86_64/intel/cascadelake/software/magma/2.7.2-foss-2023a-CUDA-12.1.1. You need to reconfigure your installation to target the correct location. (took 0 secs)

This error message should probably check if EESSI_ACCELERATOR_INSTALL is set and if it is not, give a heads up to use it?