Add node stats effective watermark thresholds docs (#107668)

Relates https://github.com/elastic/elasticsearch/pull/107244

docs/reference/cluster/nodes-stats.asciidoc

@@ -1853,6 +1853,68 @@ store.
 (integer)
 Total number of bytes available to this Java virtual machine on this file
 store.
+
+`low_watermark_free_space`::
+(<<byte-units,byte value>>)
+The effective low disk watermark for this data path on this node: when a node
+has less free space than this value for at least one data path, its disk usage
+has exceeded the low watermark. See <<disk-based-shard-allocation>> for more
+information about disk watermarks and their effects on shard allocation.
+
+`low_watermark_free_space_in_bytes`::
+(integer)
+The effective low disk watermark, in bytes, for this data path on this node:
+when a node has less free space than this value for at least one data path, its
+disk usage has exceeded the low watermark. See <<disk-based-shard-allocation>>
+for more information about disk watermarks and their effects on shard
+allocation.
+
+`high_watermark_free_space`::
+(<<byte-units,byte value>>)
+The effective high disk watermark for this data path on this node: when a node
+has less free space than this value for at least one data path, its disk usage
+has exceeded the high watermark. See <<disk-based-shard-allocation>> for more
+information about disk watermarks and their effects on shard allocation.
+
+`high_watermark_free_space_in_bytes`::
+(integer)
+The effective high disk watermark, in bytes, for this data path on this node:
+when a node has less free space than this value for at least one data path, its
+disk usage has exceeded the high watermark. See <<disk-based-shard-allocation>>
+for more information about disk watermarks and their effects on shard
+allocation.
+
+`flood_stage_free_space`::
+(<<byte-units,byte value>>)
+The effective flood stage disk watermark for this data path on this node: when
+a node has less free space than this value for at least one data path, its disk
+usage has exceeded the flood stage watermark. See
+<<disk-based-shard-allocation>> for more information about disk watermarks and
+their effects on shard allocation.
+
+`flood_stage_free_space_in_bytes`::
+(integer)
+The effective flood stage disk watermark, in bytes, for this data path on this
+node: when a node has less free space than this value for at least one data
+path, its disk usage has exceeded the flood stage watermark. See
+<<disk-based-shard-allocation>> for more information about disk watermarks and
+their effects on shard allocation.
+
+`frozen_flood_stage_free_space`::
+(<<byte-units,byte value>>)
+The effective flood stage disk watermark for this data path on a dedicated
+frozen node: when a dedicated frozen node has less free space than this value
+for at least one data path, its disk usage has exceeded the flood stage
+watermark. See <<disk-based-shard-allocation>> for more information about disk
+watermarks and their effects on shard allocation.
+
+`frozen_flood_stage_free_space_in_bytes`::
+(integer)
+The effective flood stage disk watermark, in bytes, for this data path on a
+dedicated frozen node: when a dedicated frozen node has less free space than
+this value for at least one data path, its disk usage has exceeded the flood
+stage watermark. See <<disk-based-shard-allocation>> for more information about
+disk watermarks and their effects on shard allocation.
 =======
 
 `io_stats` (Linux only)::