Updates to 21.1 session variables

[ericharmeling]

This PR adds the following session variables:

• locality (added to 21.1, 20.2, and 20.1; not sure why this was left out in previous releases)
• default_transaction_use_follower_reads (related to sql: introduce connection/session variable to enable follower reads #9584; will introduce follow-up PR)
• enable_drop_enum_value (related to release-21.1: sql: move ALTER TYPE ... DROP VALUE behind a feature flag #10137)

Note the following session variables, which exist in v21.1.0-beta.2, but remain undocumented:

• disable_partially_distributed_plans (for internal testing - no docs need; sql: some changes around experimental distsql planning cockroach#50903)
• override_multi_region_zone_config (not sure if we need docs for this @rmloveland; release-21.1: sql: Change to using a session variable instead of FORCE syntax for overriding zone configuration settings for MR databases and tables cockroach#62119, sql: Require FORCE to modify protected zone config fields #10131)
• testing_vectorize_inject_panics (for internal testing - no docs need; roachtest: temporarily omit testing setting in sqlsmith cockroach#55516)
• enable_experimental_stream_replication (not sure if we need docs for this @jseldess ; streamingccl: add experimental setting to guard the replication features #10110; streamingccl: add experimental setting to guard the replication features cockroach#60826)
• experimental_distsql_planning (we never documented this for 20.2, but I'm not sure if we need docs @yuzefovich)
• experimental_enable_implicit_column_partitioning (not sure if we need docs for this @rmloveland; sql: add base functionality to create implicit index column partitioning cockroach#58521, sql: allow REGIONAL BY ROW without experimental flag cockroach#60465)
• experimental_enable_unique_without_index_constraints (this needs docs @ianjevans; sql: add unique constraints to table descriptor for UNIQUE WITHOUT INDEX cockroach#57666)
• experimental_use_new_schema_changer (I don't think this needs docs; cockroachdb/cockroach@c0bb7c0#diff-cac22f70970154948cf8958f45e3fb5946f5dfb71a96aac384e99ccf310119b1)

Just mentioning people in this description for visibility. No response required. :)

[cockroach-teamcity]

This change is 

[cockroach-teamcity]

Online preview: http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/8499df55525614168bf202d7c63a720302ab52ce/

Edited pages:

• v20.2/experimental-features.md
• v21.1/experimental-features.md

[rmloveland]

Eric Harmeling ***@***.***> writes:

* override_multi_region_zone_config (not sure if we need docs for this @rmloveland; cockroachdb/cockroach#62119, #10131)

This will def get docs. Some discussion already in #10131 since it appears in the beta.2 release (and thus has a release note and will get doc'd etc.)

* experimental_enable_implicit_column_partitioning (not sure if we need docs for this @rmloveland; cockroachdb/cockroach#58521, cockroachdb/cockroach#60465)

Looks like it has no release note so I'll trust the PR committer on that. :-} (Esp. since it is "experimental" and thus (IMO) best avoided by the masses ;-} )

Just mentioning people in this description for visibility. No response

required. :) If you mention Beetlejuice 3 times (er, 2 in this case) he will appear :-D

[cockroach-teamcity]

Online preview: http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/5d39a2ce31dd64c3c13ff87ee719dfba18ad3ea4/

[cockroach-teamcity]

Online preview: http://cockroach-docs-review.s3-website-us-east-1.amazonaws.com/94de3a9eb2d52f9893b1804b07c94162229b4e50/