Incorrect documentation of `misa` CSR in User Manual

There are two issues related to misa documentation:

1. The "access type" should be RO, not RW.
2. For CV32A6* the "reset value" should be: {2'b01, 4'b0000, 26'b00_1001_0100_0001_0001_0000_0100} or 0x4094_1104. That is MXL=32-bits, Non-standard instructions, User mode, Supervisor mode and MIC ISA.

Note: I could be wrong about the power-on-reset value of misa, but for sure it is not 0x0.