docs: document GIT_EUSER/GIT_EPASSTHROUGH

docs/error-handling.md

@@ -110,6 +110,15 @@ int git_repository_open(git_repository **repository, const char *path)
 }
 ~~~
 
+Note that some error codes have been defined with a specific meaning in the
+context of callbacks:
+- `GIT_EUSER` provides a way to bubble up a non libgit2-related failure, which
+  allows it to be preserved all the way up to the initial function call (a `git_cred`
+  setup trying to access an unavailable LDAP server for instance).
+- `GIT_EPASSTHROUGH` provides a way to tell libgit2 that it should behave as if
+  no callback was provided. This is of special interest to bindings, which would
+  always provide a C function as a "trampoline", and decide at runtime what to do.
+
 The public error API
 --------------------