Update readBytesUntil.adoc

Language/Functions/Communication/Serial/readBytesUntil.adoc

@@ -14,9 +14,9 @@ title: Serial.readBytesUntil()
 
 [float]
 === Description
-Serial.readBytesUntil() reads characters from the serial buffer into an array. The function terminates if the terminator character is detected, the determined length has been read, or it times out (see link:../settimeout[Serial.setTimeout()]). The function returns the characters up to the last character before the supplied terminator. The terminator itself is not returned in the buffer.
+Serial.readBytesUntil() reads characters from the serial buffer into an array. The function terminates (checks being done in this order) if the determined length has been read, if it times out (see link:../settimeout[Serial.setTimeout()]), or if the terminator character is detected (in which case the function returns the characters up to the last character before the supplied terminator). The terminator itself is not returned in the buffer.
 
-`Serial.readBytesUntil()` returns the number of characters read into the buffer. A 0 means no valid data was found.
+`Serial.readBytesUntil()` returns the number of characters read into the buffer. A 0 means that the `length` parameter \<= 0, a time out occurred before any other input, or a termination character was found before any other input.
 
 `Serial.readBytesUntil()` inherits from the link:../../stream[Stream] utility class.
 [%hardbreaks]
@@ -49,7 +49,7 @@ Data type: `size_t`.
 
 [float]
 === Notes and Warnings
-The terminator character is discarded from the serial buffer.
+The terminator character is discarded from the serial buffer, unless the number of characters read and copied into the buffer equals `length`.
 [%hardbreaks]
 
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