Improve wording of environment variables

[causten]

In PR https://github.com/ROCm/AMDMIGraphX/pull/3568 it was noted that the sentence

Set to "1", "enable", "enabled", "yes", or "true" to use.

It is not specific enough and anyone running it through a translator could cause confusion.

This issue has been opened to go through the docs/dev/env_vars.rst document and improve the language used