Circumstances forced me to finally learn what the BCRSMatrix 'implicit'
build mode really does, and where its limitations come from.  I had
to read the code for that.  As it turns out, the documentation of the
mode is quite misleading.  My main gripe is that the code uses two
different auxiliary data structures, but the documentation calls them
both 'overflow'.  This problem even extends to the variable naming
in the code.  Behold: the parameter 'overflowsize' is NOT the size
of the member 'overflow'.  It doesn't even have anything to do with
that member...

To improve the situation, this patch does three things:
a) It rewrites the documentation.  In particular, the 'overflow area'
   is now clearly distinguished from the 'compression buffer'.
   The latter is a new word I introduce.
b) It renames the BCRSMatrix method parameter _overflowsize to
   compressionBufferSize, because that is what it is:  That parameter
   has nothing to do with the 'overflow' data member, or even the
   concept of 'overflowing' in general.
c) It renames the exception 'ImplicitModeOverflowExhausted' to
   'ImplicitModeCompressionBufferExhausted', for the same reason.
   This is the only interface changes.  The code keeps the old
   exception for backward-compatibility, but makes it trigger a
   deprecation warning.