[MINOR][SQL][DOCS] Improve unix_timestamp's scaladoc (and typo hunting)

## What changes were proposed in this pull request?

* Docs are consistent (across different `unix_timestamp` variants and their internal expressions)
* typo hunting

## How was this patch tested?

local build

Author: Jacek Laskowski <jacek@japila.pl>

Closes apache#17801 from jaceklaskowski/unix_timestamp.

Author: jaceklaskowski

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/datetimeExpressions.scala

@@ -488,7 +488,7 @@ case class DateFormatClass(left: Expression, right: Expression, timeZoneId: Opti
  * Deterministic version of [[UnixTimestamp]], must have at least one parameter.
  */
 @ExpressionDescription(
-  usage = "_FUNC_(expr[, pattern]) - Returns the UNIX timestamp of the give time.",
+  usage = "_FUNC_(expr[, pattern]) - Returns the UNIX timestamp of the given time.",
   extended = """
     Examples:
       > SELECT _FUNC_('2016-04-08', 'yyyy-MM-dd');
@@ -1225,8 +1225,8 @@ case class ParseToTimestamp(left: Expression, format: Expression, child: Express
   extends RuntimeReplaceable {
 
   def this(left: Expression, format: Expression) = {
-  this(left, format, Cast(UnixTimestamp(left, format), TimestampType))
-}
+    this(left, format, Cast(UnixTimestamp(left, format), TimestampType))
+  }
 
   override def flatArguments: Iterator[Any] = Iterator(left, format)
   override def sql: String = s"$prettyName(${left.sql}, ${format.sql})"

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala

@@ -423,7 +423,7 @@ object DateTimeUtils {
   }
 
   /**
-   * Parses a given UTF8 date string to the corresponding a corresponding [[Int]] value.
+   * Parses a given UTF8 date string to a corresponding [[Int]] value.
    * The return type is [[Option]] in order to distinguish between 0 and null. The following
    * formats are allowed:
    *

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

@@ -2491,10 +2491,10 @@ object functions {
    * Converts a date/timestamp/string to a value of string in the format specified by the date
    * format given by the second argument.
    *
-   * A pattern could be for instance `dd.MM.yyyy` and could return a string like '18.03.1993'. All
-   * pattern letters of `java.text.SimpleDateFormat` can be used.
+   * A pattern `dd.MM.yyyy` would return a string like `18.03.1993`.
+   * All pattern letters of `java.text.SimpleDateFormat` can be used.
    *
-   * @note Use when ever possible specialized functions like [[year]]. These benefit from a
+   * @note Use specialized functions like [[year]] whenever possible as they benefit from a
    * specialized implementation.
    *
    * @group datetime_funcs
@@ -2647,7 +2647,11 @@ object functions {
   }
 
   /**
-   * Gets current Unix timestamp in seconds.
+   * Returns the current Unix timestamp (in seconds).
+   *
+   * @note All calls of `unix_timestamp` within the same query return the same value
+   * (i.e. the current timestamp is calculated at the start of query evaluation).
+   *
    * @group datetime_funcs
    * @since 1.5.0
    */
@@ -2657,7 +2661,9 @@ object functions {
 
   /**
    * Converts time string in format yyyy-MM-dd HH:mm:ss to Unix timestamp (in seconds),
-   * using the default timezone and the default locale, return null if fail.
+   * using the default timezone and the default locale.
+   * Returns `null` if fails.
+   *
    * @group datetime_funcs
    * @since 1.5.0
    */
@@ -2666,13 +2672,15 @@ object functions {
   }
 
   /**
-   * Convert time string with given pattern
-   * (see [http://docs.oracle.com/javase/tutorial/i18n/format/simpleDateFormat.html])
-   * to Unix time stamp (in seconds), return null if fail.
+   * Converts time string with given pattern to Unix timestamp (in seconds).
+   * Returns `null` if fails.
+   *
+   * @see <a href="http://docs.oracle.com/javase/tutorial/i18n/format/simpleDateFormat.html">
+   * Customizing Formats</a>
    * @group datetime_funcs
    * @since 1.5.0
    */
-  def unix_timestamp(s: Column, p: String): Column = withExpr {UnixTimestamp(s.expr, Literal(p)) }
+  def unix_timestamp(s: Column, p: String): Column = withExpr { UnixTimestamp(s.expr, Literal(p)) }
 
   /**
    * Convert time string to a Unix timestamp (in seconds).