[SPARK-31306][DOCS] update rand() function documentation to indicate exclusive upper bound

### 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](https://github.com/apache/spark/blob/a1dbcd13a3eeaee50cc1a46e909f9478d6d55177/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 apache#28071 from Smeb/master.

Authored-by: Ben Ryves <[email protected]>
Signed-off-by: HyukjinKwon <[email protected]>

Author: Smeb

R/pkg/R/functions.R

@@ -2975,7 +2975,7 @@ setMethod("lpad", signature(x = "Column", len = "numeric", pad = "character"),
 
 #' @details
 #' \code{rand}: Generates a random column with independent and identically distributed (i.i.d.)
-#' samples from U[0.0, 1.0].
+#' samples uniformly distributed in [0.0, 1.0).
 #' Note: the function is non-deterministic in general case.
 #'
 #' @rdname column_nonaggregate_functions

python/pyspark/sql/functions.py

@@ -652,7 +652,7 @@ def percentile_approx(col, percentage, accuracy=10000):
 @since(1.4)
 def rand(seed=None):
     """Generates a random column with independent and identically distributed (i.i.d.) samples
-    from U[0.0, 1.0].
+    uniformly distributed in [0.0, 1.0).
 
     .. note:: The function is non-deterministic in general case.
 

sql/core/src/main/scala/org/apache/spark/sql/functions.scala

@@ -1227,7 +1227,7 @@ object functions {
 
   /**
    * Generate a random column with independent and identically distributed (i.i.d.) samples
-   * from U[0.0, 1.0].
+   * uniformly distributed in [0.0, 1.0).
    *
    * @note The function is non-deterministic in general case.
    *
@@ -1238,7 +1238,7 @@ object functions {
 
   /**
    * Generate a random column with independent and identically distributed (i.i.d.) samples
-   * from U[0.0, 1.0].
+   * uniformly distributed in [0.0, 1.0).
    *
    * @note The function is non-deterministic in general case.
    *