ACTION-1507/ISSUE-675: Modify readonly mapping for AXValue on AXAPI.

Replaced all occurrences of:
"AXValue (not writeable)"

with:
"AXValue:  the accessibilityIsAttributeSettable method returns NO."

and all occurrences of:
"AXValue (writable)"

with:
"AXValue:  the accessibilityIsAttributeSettable method returns YES."