Touch-ups to GC config knobs

[gewarren]

• touch ups to core config knobs (feedback from PR Add run-time GC settings #15844)
• also adds new page for GCLOHThreshold option for app.config

Preview link: https://review.docs.microsoft.com/en-us/dotnet/core/run-time-config/garbage-collector?branch=pr-en-us-15914

[Maoni0]

LGTM! BTW, I apologize - I forgot to mention that for the GCLOHThreshold config we should note that you can only specify a value that's larger than the default (which is 85000 bytes).

[mairaw]

LGTM modulo @Maoni0's comment

[gewarren]

Thanks all for the reviews.