[BUGFIX] FindMKL.cmake fix comment and cmake variables and search paths

[sfraczek]

Description

Fixes to small bugs in FindMKL.cmake explained here: #19151

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)
• Code is well-documented

Comments

Regarding backwards compatibility: If someone previously used MKL_ROOT environment variable set manually because MKLROOT wasn't working, then this change may break things for them if they don't have MKLROOT env var set (which is done by sourcing mklvars.sh by the book https://software.intel.com/content/www/us/en/develop/documentation/mkl-linux-developer-guide/top/getting-started/setting-environment-variables/scripts-to-set-environment-variables.html).

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[leezu]

Thank you. Renaming MKL_ROOT to MKLROOT for the env variable check is not backwards compatible, but common MKLROOT is common whereas MKL_ROOT isn't