doxy: fix conditional usage, ref warnings

doxygen < 1.7.? seems to have been more tolerant of single line
\if/\endif

This change fixes warnings such as:
mainpage.dox:13: warning: unable to resolve reference to `vp8_encoder-'
for \ref command
vpx_decoder.h:193: warning: explicit link request to 'n' could not be
resolved

Change-Id: If3d04af5ede1b0d1e2c63021d0e4ac8f98db20b2

usage.dox

@@ -1,6 +1,6 @@
 /*!\page usage Usage
 
-    The vpx Multi-Format codec SDK provides a unified interface amongst its
+    The vpx multi-format codec SDK provides a unified interface amongst its
     supported codecs. This abstraction allows applications using this SDK to
     easily support multiple video formats with minimal code duplication or
     "special casing." This section describes the interface common to all codecs.
@@ -14,8 +14,12 @@
 
     Fore more information on decoder and encoder specific usage, see the
     following pages:
-    \if decoder - \subpage usage_decode \endif
-    \if decoder - \subpage usage_encode \endif
+    \if decoder
+    - \subpage usage_decode
+    \endif
+    \if decoder
+    - \subpage usage_encode
+    \endif
 
     \section usage_types Important Data Types
     There are two important data structures to consider in this interface.