Doc updates on LOCATE and CONCAT edge cases.

Author: Andras Palinkas

docs/reference/sql/functions/string.asciidoc

@@ -107,7 +107,7 @@ CONCAT(
 
 *Output*: string
 
-*Description*: Returns a character string that is the result of concatenating `string_exp1` to `string_exp2`. If one of the string is `NULL`, the other string will be returned.
+*Description*: Returns a character string that is the result of concatenating `string_exp1` to `string_exp2`. `NULL` input strings are treated as empty strings.
 
 [source, sql]
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@@ -228,7 +228,7 @@ LOCATE(
 
 *Output*: integer
 
-*Description*: Returns the starting position of the first occurrence of `pattern` within `source`. The search for the first occurrence of `pattern` begins with the first character position in `source` unless the optional argument, `start`, is specified. If `start` is specified, the search begins with the character position indicated by the value of `start`. The first character position in `source` is indicated by the value 1. If `pattern` is not found within `source`, the value 0 is returned.
+*Description*: Returns the starting position of the first occurrence of `pattern` within `source`. The optional `start` specifies the character position to start the search with. The first character position in `source` is indicated by the value 1. Not specifying `start` or specifying it as `NULL`, any negative value, 0 or 1, starts the search at the first character position. If `pattern` is not found within `source`, the value 0 is returned.
 
 [source, sql]
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