Document time unit limitations for date histograms

Adding some allowed abbreviated values for intervals in date histograms
as well as documenting the limitations of intervals larger than days.

Closes elastic#23294

docs/reference/aggregations/bucket/datehistogram-aggregation.asciidoc

@@ -27,11 +27,13 @@ POST /sales/_search?size=0
 // CONSOLE
 // TEST[setup:sales]
 
-Available expressions for interval: `year`, `quarter`, `month`, `week`, `day`, `hour`, `minute`, `second`
+Available expressions for interval: `year` (`1y`), `quarter` (`1q`), `month` (`1M`), `week` (`1w`),
+`day` (`1d`), `hour` (`1h`), `minute` (`1m`), `second` (`1s`)
 
 Time values can also be specified via abbreviations supported by <<time-units,time units>> parsing.
 Note that fractional time values are not supported, but you can address this by shifting to another
-time unit (e.g., `1.5h` could instead be specified as `90m`).
+time unit (e.g., `1.5h` could instead be specified as `90m`). Also note that time intervals larger than
+than days do not support arbitrary values but can only be one unit large (e.g. `1y` is valid, `2y` is not).
 
 [source,js]
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