Reinstate missing documentation

[DaveCTurner]

The setting index.routing.allocation.enable was documented in v1.7 but lost in the refactoring in f123a53 so has been undocumented since 2.0.

It might also be worth checking if any other settings were lost in the same change. I have not done so yet.

/cc @elastic/es-distributed.

[DaveCTurner]

The following settings appear on lines removed in f123a53 and not in the docs/ folder of master:

index.cache.filter.expire
index.cache.filter.max_size
index.gateway.snapshot_interval
index.gc_deletes
index.recovery.initial_shards
index.routing.allocation.disable_allocation
index.routing.allocation.disable_new_allocation
index.routing.allocation.disable_replica_allocation
index.routing.allocation.enable
index.routing.rebalance.enable
index.translog.disable_flush
index.translog.flush_threshold_ops
index.translog.flush_threshold_period
index.translog.fs.type
index.ttl.disable_purge
index.warmer.enabled
indices.cache.filter.size
indices.recovery.compress
indices.recovery.concurrent_small_file_streams
indices.recovery.concurrent_streams
indices.recovery.file_chunk_size
indices.recovery.translog_ops
indices.recovery.translog_size
indices.ttl.interval

Of these, the following still seem to be valid settings:

index.gc_deletes
index.routing.allocation.enable
index.routing.rebalance.enable
index.ttl.disable_purge
index.warmer.enabled

The following do not seem to be valid settings, but are still mentioned in a test:

indices.recovery.concurrent_streams
indices.recovery.concurrent_small_file_streams

elasticsearch/server/src/test/java/org/elasticsearch/indices/recovery/RecoverySourceHandlerTests.java

Lines 104 to 105 in 5904d93

	Settings settings = Settings.builder().put("indices.recovery.concurrent_streams", 1).
	put("indices.recovery.concurrent_small_file_streams", 1).build();

[DaveCTurner]

@Sue-Gallagher @debadair I'm happy to try and resurrect these lost docs, but I'm not sure where in the docs tree they should go, as things seem to have moved around quite a lot since 1.7. Your guidance would be appreciated.

[debadair]

It looks like the missing settings should be added to https://www.elastic.co/guide/en/elasticsearch/reference/6.2/index-modules.html#index-modules-settings.

If you want to open a PR for that, that would be most awesome.

[DaveCTurner]

I reinstated docs for index.gc_deletes, index.routing.allocation.enable and index.routing.rebalance.enable here.

Setting index.ttl.disable_purge is unused so I opened #29526 and #29527 to remove it.

Setting index.warmer.enabled requires knowledge that I do not have so I opened #29529 about this one.

[ywelsch]

LGTM