[kernel-doc] fix various DocBook build problems/warnings

Most serious is fixing include/sound/pcm.h, which breaks the DocBook
build.

The other stuff is just filling in things that cause warnings.
diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index fa23ebf..389e8eb 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -903,7 +903,7 @@
 int snd_pcm_format_linear(snd_pcm_format_t format);
 int snd_pcm_format_little_endian(snd_pcm_format_t format);
 int snd_pcm_format_big_endian(snd_pcm_format_t format);
-/**
+/*
  * snd_pcm_format_cpu_endian - Check the PCM format is CPU-endian
  * @format: the format to check
  *