### What changes were proposed in this pull request?
A small documentation change to clarify that the `rand()` function produces values in `[0.0, 1.0)`.
### Why are the changes needed?
`rand()` uses `Rand()` - which generates values in [0, 1) ([documented here](a1dbcd13a3/sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/randomExpressions.scala (L71))). The existing documentation suggests that 1.0 is a possible value returned by rand (i.e for a distribution written as `X ~ U(a, b)`, x can be a or b, so `U[0.0, 1.0]` suggests the value returned could include 1.0).
### Does this PR introduce any user-facing change?
Only documentation changes.
### How was this patch tested?
Documentation changes only.
Closes#28071 from Smeb/master.
Authored-by: Ben Ryves <benjamin.ryves@getyourguide.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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