[Doc] Clarify that session can also mean a ray cluster

[jjyao]

Why are these changes needed?

Clarify in the glossary that session can also mean a ray cluster

Related issue number

Closes #35093

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[scottsun94]

Can we use bullets points to clearly list the different meanings? Something like:

• A Ray Train/Tune session, ....
• A Ray Cluster instance,...

It's a bit hard for me to digest it when there are several paragraphs.