network: doc and code typo fixes (#36422)

This fixes some typos in the documentation, both grammar as well as
incorrect field names.
It also changes the casing of CheckSum to Checksum in L2TP to match
other casings.

test/test-network/conf/25-l2tp-udp.netdev

@@ -12,8 +12,8 @@ Local=static@test1
 Remote=192.168.30.101
 EncapsulationType=udp
 UDPCheckSum=true
-UDP6CheckSumRx=true
-UDP6CheckSumTx=true
+UDP6ChecksumRx=true
+UDP6ChecksumTx=true
 
 [L2TPSession]
 SessionId=15