Fix comment when build with -Wdocumentation and Clang

./includes/nghttp2/nghttp2.h:70:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration
./includes/nghttp2/nghttp2.h:1536:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration
./includes/nghttp2/nghttp2.h:2724:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration
./includes/nghttp2/nghttp2.h:2850:5: error: '@struct' command should not be used in a comment attached to a non-struct declaration
Extend namelen and valuelen in nghttp2_nv to size_t
Fix GCC when use -Wpedantic

nghttp2_int.h:50:33: error: comma at end of enumerator list [-Werror=pedantic]
Fix GCC when use -Wpedantic

nghttp2_hd.h:115:30: error: comma at end of enumerator list [-Werror=pedantic]
Fix GCC when use -Wpedantic

nghttp2_stream.h:94:43: error: comma at end of enumerator list [-Werror=pedantic]
Fix GCC build with -Werror

nghttp2_session.c:1615:9: error: C++ style comments are not allowed in ISO C90 [-Werror]
Fix GCC build with -Werror

libevent-server.c:691:8: error: C++ style comments are not allowed in ISO C90