BIP155: clarify that "services" uses CompactSize

The Bitcoin Core source code has `VARINT` type which is different than
the "variable integer" format used all over the P2P protocol and also
for the "services" field in this BIP. The latter is called `CompactSize`
in some BIPs and also in the Bitcoin Core source code, thus use the word
`CompactSize` to refer to it and link to its documentation.

bip-0155.mediawiki

@@ -46,7 +46,7 @@ The <code>addrv2</code> message is defined as a message where <code>pchCommand =
 It is serialized in the standard encoding for P2P messages.
 Its format is similar to the current <code>addr</code> message format
 <ref>[https://bitcoin.org/en/developer-reference#addr Bitcoin Developer Reference: addr message]</ref>, with the difference that the 
-fixed 16-byte IP address is replaced by a network ID and a variable-length address, and the services format has been changed to VARINT.
+fixed 16-byte IP address is replaced by a network ID and a variable-length address, and the services format has been changed to [https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer CompactSize].
 
 This means that the message contains a serialized <code>std::vector</code> of the following structure:
 
@@ -59,9 +59,9 @@ This means that the message contains a serialized <code>std::vector</code> of th
 | <code>time</code>
 | Time that this node was last seen as connected to the network. A time in Unix epoch time format.
 |-
-| <code>VARINT</code> (unsigned)
+| <code>CompactSize</code>
 | <code>services</code>
-| Service bits. A 64-wide bit field.
+| Service bits. A bit field that is 64-bit wide, encoded in [https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer CompactSize].
 |-
 | <code>uint8_t</code>
 | <code>networkID</code>
@@ -142,7 +142,7 @@ The reference implementation is available at (to be done)
 
 ==Acknowledgements==
 
-- Jonas Schnelli: change <code>services</code> field to VARINT, to make the message more compact in the likely case instead of always using 8 bytes.
+- Jonas Schnelli: change <code>services</code> field to [https://en.bitcoin.it/wiki/Protocol_documentation#Variable_length_integer CompactSize], to make the message more compact in the likely case instead of always using 8 bytes.
 
 - Gregory Maxwell: various suggestions regarding extensibility