slog: promote subheadings

Remove the "Advanced Topics" heading and promote the subheadings.

Change-Id: I0ca5ac9d7ec726cd56072528968c634ab290ce30
Reviewed-on: https://go-review.googlesource.com/c/exp/+/476200
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>

slog/doc.go

diff --git a/slog/doc.go b/slog/doc.go
index 7b9133f..3b37eec 100644
--- a/slog/doc.go
+++ b/slog/doc.go
@@ -212,9 +212,7 @@
 
 It is recommended to pass a context to an output method if one is available.
 
-# Advanced topics
-
-Customizing a type's logging behavior.
+# Customizing a type's logging behavior
 
 If a type implements the [LogValuer] interface, the [Value] returned from its LogValue
 method is used for logging. You can use this to control how values of the type
@@ -226,7 +224,7 @@
 method handles these cases carefully, avoiding infinite loops and unbounded recursion.
 Handler authors and others may wish to use Value.Resolve instead of calling LogValue directly.
 
-Wrapping output methods.
+# Wrapping output methods
 
 The logger functions use reflection over the call stack to find the file name
 and line number of the logging call within the application. This can produce
@@ -248,7 +246,7 @@
 The Infof function in the package-level example called "wrapping"
 demonstrates how to do this.
 
-Working with Records.
+# Working with Records
 
 Sometimes a Handler will need to modify a Record
 before passing it on to another Handler or backend.
@@ -262,7 +260,7 @@
 or create a new Record with [NewRecord]
 and build up its Attrs by traversing the old ones with [Record.Attrs].
 
-Performance considerations.
+# Performance considerations
 
 If profiling your application demonstrates that logging is taking significant time,
 the following suggestions may help.