cmd/compile: minor doc enhancements

Add a little more detail to the ssa README relating to GOSSAFUNC.

Update the -d=ssa help section to give a little more detail on what
to expect with applying the /debug=X qualifier to a phase.

Change-Id: I7027735f1f2955dbb5b9be36d9a648e8dc655048
Reviewed-on: https://go-review.googlesource.com/c/go/+/315229
Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/ssa/README.md

@@ -184,6 +184,19 @@ compile passes, making it easy to see what each pass does to a particular
 program. You can also click on values and blocks to highlight them, to help
 follow the control flow and values.
 
+The value specified in GOSSAFUNC can also be a package-qualified function
+name, e.g. 
+
+	GOSSAFUNC=blah.Foo go build
+    
+This will match any function named "Foo" within a package whose final
+suffix is "blah" (e.g. something/blah.Foo, anotherthing/extra/blah.Foo).
+
+If non-HTML dumps are needed, append a "+" to the GOSSAFUNC value
+and dumps will be written to stdout:
+
+	GOSSAFUNC=Bar+ go build
+
 <!---
 TODO: need more ideas for this section
 -->

src/cmd/compile/internal/ssa/compile.go

@@ -297,6 +297,11 @@ enables time reporting for all phases
     -d=ssa/prove/debug=2
 sets debugging level to 2 in the prove pass
 
+Be aware that when "/debug=X" is applied to a pass, some passes
+will emit debug output for all functions, and other passes will
+only emit debug output for functions that match the current
+GOSSAFUNC value.
+
 Multiple flags can be passed at once, by separating them with
 commas. For example: