[sram_ctrl/doc] Execution from SRAM clarifications
[sram_ctrl/doc] Execution from SRAM clarifications

This adds more details regarding the execution from SRAM feature to the
docs.

See also #8969

Signed-off-by: Michael Schaffner <a005a2d3342a687ea2d05360c953107acbe83e08@opentitan.org>

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