fix: descriptions of some standard database functions (#1519)

after trying them out on my own, I saw some derivations from the actual
behavior compared to the described behavior.

---------

Co-authored-by: René Jeglinsky <[email protected]>
Co-authored-by: Christian Georgi <[email protected]>

guides/databases.md

@@ -375,12 +375,14 @@ This set of functions are by large the same as specified in OData:
 * `floor(x)` — rounds the input numeric parameter down to the nearest numeric value
 * `round(x)` — rounds the input numeric parameter to the nearest numeric value.
                The mid-point between two integers is rounded away from zero, i.e. 0.5 is rounded to 1 and ‑0.5 is rounded to -1.
-* `year(x)` `month(x)`, `day(x)`, `hour(x)`, `minute(x)`, `second(x)`, `fractionalseconds(x)`, `time(x)`, `date(x)` —
+* `year(x)` `month(x)`, `day(x)`, `hour(x)`, `minute(x)`, `second(x)` —
   returns parts of a datetime for a given `cds.DateTime` / `cds.Date` / `cds.Time`
+* `time(x)`, `date(x)` - returns a string representing the `time` / `date` for a given `cds.DateTime` / `cds.Date` / `cds.Time`
+* `fractionalseconds(x)` - returns a a `Decimal` representing the fractions of a second for a given `cds.Timestamp`
 * `maxdatetime(x)`, `mindatetime(x)` — return the maximum or minimum datetime for a given `cds.DateTime` / `cds.Date` / `cds.Time`
-* `totalseconds(x)` — returns the total seconds of a datetime for a given `cds.DateTime` / `cds.Time`
+* `totalseconds(x)` — returns the duration of the value in total seconds, including fractional seconds. The [OData spec](https://docs.oasis-open.org/odata/odata/v4.01/odata-v4.01-part2-url-conventions.html#sec_totalseconds) defines the input as EDM.Duration: `P12DT23H59M59.999999999999S`
 * `now()` — returns the current datetime
-* `min(x)` `max(x)` `sum(x)` `avg(x)` `count(x)`, `countdistinct(x)` — aggregate functions
+* `min(x)` `max(x)` `sum(x)` `average(x)` `count(x)`, `countdistinct(x)` — aggregate functions
 * `search(xs,y)` — checks whether `y` is contained in any of `xs`, may be fuzzy → [see Searching Data](../guides/providing-services#searching-data)
 * `session_context(v)` — with standard variable names → [see Session Variables](#session-variables)
 > <sup>1</sup> These functions work zero-based.  E.g., `substring('abcdef', 1, 3)` returns 'bcd'