Inconsistent usage of terms RPL root and 6LBR

[Mališa Vučinić]

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Pascal Thubert wrote (https://mailarchive.ietf.org/arch/msg/6tisch/foP80MdcioJExxsy9blOnX89NUY):

“The JRC can be co-located on the 6LBR.  In this special case, the
   IPv6 address of the JRC can be omitted from the Join Response message
   for space optimization.  The 6LBR then MUST set the DODAGID field in
   the RPL DIOs [RFC6550] to its IPv6 address.  The pledge learns the
   address of the JRC once joined and upon the reception of the first
   RPL DIO message, and uses it to operate as a JP.”

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Note that the expectation is that the 6LBR is the RPL root as suggested in the 6TiSCH architecture.

When they are not the same box I expect the all the text about 6LBR throughout this doc is really about the RPL root.

This should be indicated somewhere.