[gdbremote] Document MultiMemRead packet in protocol extensions (llvm#162675)

This adds a specification for the new packet discussed in the RFC [1].

[1]:
https://discourse.llvm.org/t/rfc-a-new-vectorized-memory-read-packet/88441/12

(cherry picked from commit e91786a)

Author: felipepiovezan

lldb/docs/resources/lldbgdbremote.md

@@ -735,6 +735,57 @@ This is a performance optimization, which speeds up debugging by avoiding
 multiple round-trips for retrieving thread information. The information from this
 packet can be retrieved using a combination of `qThreadStopInfo` and `m` packets.
 
+### MultiMemRead
+
+Read memory from multiple memory ranges.
+
+This packet has one argument:
+
+* `ranges`: a list of pairs of numbers, formatted in base-16. Each pair is
+separated by a `,`, as is each number in each pair. The first number of the
+pair denotes the base address of the memory read, the second denotes the number
+of bytes to be read. The list must end with a `;`.
+
+The reply packet starts with a comma-separated list of numbers formatted in
+base-16, denoting how many bytes were read from each range, in the same order
+as the request packet. The list is followed by a `;`, followed by a sequence of
+bytes containing binary encoded data for all memory that was read. The length
+of the binary encodeed data, after being decoded as required by the GDB remote
+protocol, must be equal to the sum of the numbers provided at the start of the
+reply. The order of the binary data is the same as the order of the ranges in
+the request packet.
+
+If some part of a range is not readable, the stub may perform a partial read of
+a prefix of the range. In other words, partial reads will only ever be from the
+start of the range, never the middle or end. Support for partial reads depends
+on the debug stub.
+
+If, by applying the rules above, the stub has read zero bytes from a range, it
+must return a length of zero for that range in the reply packet; no bytes for
+this memory range are included in the sequence of bytes that follows.
+
+A stub that supports this packet must include `MultiMemRead+` in the reply to
+`qSupported`.
+
+```
+send packet: $MultiMemRead:ranges:100a00,4,200200,a0,400000,4;
+read packet: $4,0,2;<binary encoding of abcd1000><binary encoding of eeff>
+```
+
+In the example above, the first read produced `abcd1000`, the read of `a0`
+bytes from address `200200` failed to read any bytes, and the third read
+produced two bytes – `eeff` – out of the four requested.
+
+```
+send packet: $MultiMemRead:ranges:100a00,0;
+read packet: $0;
+```
+
+In the example above, a read of zero bytes was requested.
+
+**Priority to Implement:** Only required for performance, the debugger will
+fall back to doing separate read requests if this packet is unavailable.
+
 ## QEnvironment:NAME=VALUE
 
 Setup the environment up for a new child process that will soon be