doc: add definitions for tainted

Author: Boquan Fang

docs/DEVELOPMENT-GUIDE.md

@@ -347,7 +347,8 @@ The `high_water_mark` tracks the furthermost byte which has been written but not
 Note that this may be past the `write_cursor` if `s2n_stuffer_rewrite()` has been called.
 Explicitly tracking the `high_water_mark` allows us to track the bytes which need to be wiped, and helps avoids needless zeroing of memory.
 The next two bits of state track whether a stuffer was dynamically allocated (and so should be free'd later) and whether or not it is growable.
-`tainted` is set to 1 whenever the raw access functions are called.
+`tainted` is set to true whenever the raw access functions are called.
+Raw access functions return an outstanding pointer that references the beginning of a memory chunk within the stuffer. Whenever such an outstanding pointer is accessible, the stuffer's `tainted` flag must be set to true. However, if the outstanding pointer is no longer accessable, the stuffer's `tainted` flag can be set to false.
 If a stuffer is currently tainted then it can not be resized and it becomes ungrowable.
 This is reset when a stuffer is explicitly wiped, which begins the life-cycle anew.
 So any pointers returned by the raw access functions are legal only until `s2n_stuffer_wipe()` is called.