Update docs

Author: kanoshiou

docs/reference/esql/functions/description/date_trunc.asciidoc

@@ -135,7 +135,7 @@ another in which the bucket size is provided directly (two parameters).
                 If the desired bucket size is known in advance, simply provide it as the second
                 argument, leaving the range out:""", file = "bucket", tag = "docsBucketWeeklyHistogramWithSpan", explanation = """
                 NOTE: When providing the bucket size as the second parameter, it must be a time
-                duration or date period."""),
+                duration or date period. Also the reference is epoch, which starts at `0001-01-01T00:00:00Z`."""),
             @Example(
                 description = "`BUCKET` can also operate on numeric fields. For example, to create a salary histogram:",
                 file = "bucket",

docs/reference/esql/functions/examples/bucket.asciidoc

@@ -63,7 +63,7 @@ public interface DateTruncFactoryProvider {
 
     @FunctionInfo(
         returnType = { "date", "date_nanos" },
-        description = "Rounds down a date to the closest interval.",
+        description = "Rounds down a date to the closest interval since epoch, which starts at `0001-01-01T00:00:00Z`.",
         examples = {
             @Example(file = "date", tag = "docsDateTrunc"),
             @Example(