docs: describe failure writer

docs/failures.md

@@ -21,15 +21,33 @@ from the signal handler.
         @     0x7f892f7ef1c4 (unknown)
         @           0x4046f9 (unknown)
 
-By default, the signal handler writes the failure dump to the standard
-error. You can customize the destination by
-`#!cpp InstallFailureWriter()`.
+
+## Customizing Handler Output
+
+By default, the signal handler writes the failure dump to the standard error.
+However, it is possible to customize the destination by installing a callback
+using the `#!cpp google::InstallFailureWriter()` function. The function expects
+a pointer to a function with the following signature:
+
+``` cpp
+void YourFailureWriter(const char* message/* (1)! */, std::size_t length/* (2)! */);
+```
+
+1. The pointer references the start of the failure message.
+
+    !!! danger
+        The string is **not null-terminated**.
+
+2. The message length in characters.
+
+!!! warning "Possible overflow errors"
+    Users should not expect the `message` string to be null-terminated.
 
 ## User-defined Failure Function
 
 `FATAL` severity level messages or unsatisfied `CHECK` condition
 terminate your program. You can change the behavior of the termination
-by `InstallFailureFunction`.
+by `google::InstallFailureFunction`.
 
 ``` cpp
 void YourFailureFunction() {