doc groonga-httpd: fix wrong context

groonga_log_path and groonga_log_level can be also specified in http
and server contexts. See

    NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1

lines in

    { ngx_string("groonga_log_level"),
      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
      ngx_http_groonga_conf_set_log_level_slot,
      NGX_HTTP_LOC_CONF_OFFSET,
      0,
      NULL },

    { ngx_string("groonga_cache_limit"),
      NGX_HTTP_MAIN_CONF|NGX_CONF_TAKE1,
      ngx_conf_set_size_slot,
      NGX_HTTP_LOC_CONF_OFFSET,
      offsetof(ngx_http_groonga_loc_conf_t, cache_limit),
      NULL },

Descriptions of them should be also updated.