SERVER-6801 Document lack of character encoding support in $substr.

Author: astaple

source/reference/aggregation.txt

@@ -732,10 +732,16 @@ These operators manipulate strings within projection expressions.
 .. expression:: $substr
 
    :expression:`$substr` takes a string and two numbers. The first
-   number represents the number of characters in the string to skip,
-   and the second number specifies the number of characters to return
+   number represents the number of bytes in the string to skip,
+   and the second number specifies the number of bytes to return
    from the string.
 
+   .. note::
+
+      ::expression:`$substr` is not encoding aware and if used
+      improperly may produce a result string containing an invalid utf-8
+      character sequence.
+
 .. expression:: $toLower
 
    Takes a single string and converts that string to lowercase,