new system variables are not documented in alphabetical order

[morgo]

Error Report

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1. What is the URL/path of the document related to this issue?

https://docs.pingcap.com/tidb/dev/system-variables#tidb_opt_prefer_range_scan

2. How would you like to improve it?

Chronological order of when options were added is not useful to users (especially when such a long list). The options in this page are alphabetical up until this variable on: https://docs.pingcap.com/tidb/dev/system-variables#tidb_opt_prefer_range_scan

[TomShawn]

Addressed in #4589.