liblog: document the liblog<->logd protocol format

# liblog -> logd

The data that liblog sends to logd is represented below.

    struct {

        android_log_header_t header;

        union {

           struct {

                char     prio;

                char     tag[...];

                char     message[...];

            } string;

            struct {

                android_event_header_t event_header;

                android_event_*_t      payload[...];

            } binary;

        };

    };

The payload, excluding the header, has a max size of LOGGER_ENTRY_MAX_PAYLOAD.

## header

The header is added immediately before sending the log message to logd.

## `string` payload

The `string` part of the union is for normal buffers (main, system, radio, etc) and consists of a

single character priority, followed by a variable length null terminated string for the tag, and

finally a variable length null terminated string for the message.

This payload is used for the `__android_log_buf_write()` family of functions.

## `binary` payload

The `binary` part of the union is for binary buffers (events, security, etc) and consists of an

android_event_header_t struct followed by a variable number of android_event_*_t

(android_event_list_t, android_event_int_t, etc) structs.

If multiple android_event_*_t elements are present, then they must be in a list and the first

element in payload must be an android_event_list_t.

This payload is used for the `__android_log_bwrite()` family of functions. It is additionally used

for `android_log_write_list()` and the related functions that manipulate event lists.

# logd -> liblog

logd sends a `logger_entry` struct to liblog followed by the payload. The payload is identical to

the payloads defined above. The max size of the entire message from logd is LOGGER_ENTRY_MAX_LEN.

    return 0;

  }

  /*

   *  struct {

   *      // what we provide to socket

   *      android_log_header_t header;

   *      // caller provides

   *      union {

   *          struct {

   *              char     prio;

   *              char     payload[];

   *          } string;

   *          struct {

   *              uint32_t tag

   *              char     payload[];

   *          } binary;

   *      };

   *  };

   */

  header.tid = gettid();

  header.realtime.tv_sec = ts->tv_sec;

  header.realtime.tv_nsec = ts->tv_nsec;