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

mainpage.dox

@@ -12,8 +12,12 @@
 
   This distribution of the WebM VP8 Codec SDK includes the following support:
 
-  \if vp8_encoder    - \ref vp8_encoder   \endif
-  \if vp8_decoder    - \ref vp8_decoder   \endif
+  \if vp8_encoder
+  - \ref vp8_encoder
+  \endif
+  \if vp8_decoder
+  - \ref vp8_decoder
+  \endif
 
 
   \section main_startpoints Starting Points
@@ -24,8 +28,12 @@
   - Read the \ref samples "sample code" for examples of how to interact with the
     codec.
   - \ref codec reference
-    \if encoder - \ref encoder reference \endif
-    \if decoder - \ref decoder reference \endif
+    \if encoder
+    - \ref encoder reference
+    \endif
+    \if decoder
+    - \ref decoder reference
+    \endif
 
   \section main_support Support Options & FAQ
   The WebM project is an open source project supported by its community. For

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.

vpx/vpx_decoder.h

@@ -190,10 +190,10 @@ extern "C" {
      * time stamp) order. Frames produced will always be in PTS (presentation
      * time stamp) order.
      * If the decoder is configured with VPX_CODEC_USE_INPUT_FRAGMENTS enabled,
-     * data and data_sz can contain a fragment of the encoded frame. Fragment #n
-     * must contain at least partition #n, but can also contain subsequent
-     * partitions (#n+1 - #n+i), and if so, fragments #n+1, .., #n+i must be
-     * empty. When no more data is available, this function should be called
+     * data and data_sz can contain a fragment of the encoded frame. Fragment
+     * \#n must contain at least partition \#n, but can also contain subsequent
+     * partitions (\#n+1 - \#n+i), and if so, fragments \#n+1, .., \#n+i must
+     * be empty. When no more data is available, this function should be called
      * with NULL as data and 0 as data_sz. The memory passed to this function
      * must be available until the frame has been decoded.
      *