fmt: document that if String or Error panics, that value is printed

Fixes #5350.

R=r, minux.ma
CC=golang-dev
https://golang.org/cl/8950043

diff --git a/src/pkg/fmt/doc.go b/src/pkg/fmt/doc.go
index 3cd02d7ede45d575257c42909ba7158d24efff82..b8dd995c77b88529e0246e39bd0310ab2a0bf579 100644 (file)
--- a/src/pkg/fmt/doc.go
+++ b/src/pkg/fmt/doc.go
@@ -138,6 +138,16 @@
        by a single character (the verb) and end with a parenthesized
        description.
 
+       If an Error or String method triggers a panic when called by a
+       print routine, the fmt package reformats the error message
+       from the panic, decorating it with an indication that it came
+       through the fmt package.  For example, if a String method
+       calls panic("bad"), the resulting formatted message will look
+       like
+               %s(PANIC=bad)
+
+       The %s just shows the print verb in use when the failure
+       occurred.
 
        Scanning