[flash_ctrl] Clarify partition accessability language
[flash_ctrl] Clarify partition accessability language

- the previous flash documentation was not clear about whether information partitions could be programmed or erased.  A statement contrasting data and information partitions could reasonably be interpreted as meaning info partitions could only be read.

- the new updates make it very explicit which partitions can be accessed by whome.

- fixes #10685

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan