docs: compact: add blurb about how retention policy works (#2808)

Add some verbiage that explains when exactly do the blocks get deleted.

Closes #2782.

Signed-off-by: Giedrius Statkevičius <[email protected]>

docs/components/compact.md

@@ -49,6 +49,8 @@ Ideally, you will have equal retention set (or no retention at all) to all resol
 
 Not setting this flag, or setting it to `0d`, i.e. `--retention.resolution-X=0d`, will mean that samples at the `X` resolution level will be kept forever.
 
+Please note that blocks are only deleted after they completely "fall off" of the specified retention policy. In other words, the "max time" of a block needs to be older than the amount of time you had specified.
+
 ## Storage space consumption
 
 In fact, downsampling doesn't save you any space but instead it adds 2 more blocks for each raw block which are only slightly smaller or relatively similar size to raw block. This is required by internal downsampling implementation which to be mathematically correct holds various aggregations. This means that downsampling can increase the size of your storage a bit (~3x), but it gives massive advantage on querying long ranges.