updates from PR comments

site/content/docs/guide/plugin/policy-expr.md

@@ -46,8 +46,9 @@ UTC by including `+{HH}:[MM]` or `-{HH}:[MM]`.
 #### Span
 
 Spans represent a duration of time using the `jiff` [crate] `Span` type. Policy
-expression spans can include weeks, days, hours, minutes, and seconds (including
-decimal fractions of a second). Spans are prefixed with the letter "P" followed
+expression spans can include weeks, days, hours, minutes, and seconds. They can
+include optional decimal fractions of the smallest unit of time (hours, minutes,
+or seconds) used (e.g. `1.5h`). Spans are prefixed with the letter "P" followed
 by optional date units. Time units are separated from date units with the letter
 "T". All date and time units are specified in single case-agnostic letter
 abbreviations after the number. For example, a span of one week, one day, one
@@ -97,9 +98,10 @@ following functions:
 | `(lte <A> <B>)`| less than or equal | non-identifier primitives | evaluate `A <= B` |
 | `(eq <A> <B>)` | equal | non-identifier primitives | evaluate `A == B` |
 | `(neq <A> <B>)` | not equal | non-identifier primitives | evaluate `A != B` |
-| `(add <A> <B>)` | add | integers, floats, bools, (datetime and span) | evaluate `A + B` |
-| `(sub <A> <B>)` | subtract | integers, floats, bools, (datetime and span) | evaluate `A - B` |
+| `(add <A> <B>)` | add | integers, floats, bools, spans, or (datetime + span) | evaluate `A + B` |
+| `(sub <A> <B>)` | subtract | integers, floats, bools, spans, or (datetime + span) | evaluate `A - B` |
 | `(divz <A> <B>)` | divide or zero | integers, floats | if `B == 0` return `B`, else evaluate `A / B` |
+| `(duration <A> <B>)` | duration | datetimes | evaluate `A - B` to produce a `span` |
 | `(and <A> <B>)` | and | bools | evaluate `A & B` |
 | `(or <A> <B>)` | or | bools | evaluate `A | B` |
 | `(not <A>)` | not | bool | evaluate `!A` |