fix description of why parallel decomposition affects preconditioners

[bueler]

Wrong in Chapter 2: "These preconditioners do not communicate any information between rows at all, and thus they exhibit no dependence."

The right thing is along the lines of: "Preconditioners are fast if they avoid communication, i.e. needing info in rows they don't own. Some preconditioners (e.g. jacobi) only use info from single rows, so they are insensitive to the parallel decomposition. Better preconditioners, such as block-Jacobi and ASM, act on multiple rows owned by the processor. Different parallel decompositions therefore determine different actual preconditioners."

[bueler]

Done, in effect. See last paragraph Chapter 2.