EIP-1895: Improvements (ethereum#2153)

* EIP-1895: Better constants and address range description

Address @Arachnid's comments about presenting constants and how an address value is limited.

* EIP-1985: Fix a typo

* EIP-1985: Use "instructions" instead of "opcodes"

EIPS/eip-1985.md

@@ -25,23 +25,24 @@ by eliminating unrealistic edge cases.
 ## Specification
 
 If `block.number >= {FORK_BLOCK}`, the following value ranges are introduced.
-They restrict the results (i.e. values pushed to the stack) of the opcodes listed below.
+They restrict the results (i.e. values pushed to the stack) of the instructions listed below.
 
 1. *gas*, *gas limit*, *block gas limit*
-   is a range between `0` and `9223372036854775807` (`2**63 - 1`).
-   It affects following the opcodes:
+   is a range between `0` and `0x7fffffffffffffff` (`2**63 - 1`, `9223372036854775807`).
+   It affects the following instructions:
    - `GASLIMIT` (`0x45`),
    - `GAS` (`0x5a`).
 
 2. *block number*, *timestamp*
-   is a range between `0` and `9223372036854775807` (`2**63 - 1`).
-   It affects the following opcodes:
+   is a range between `0` and `0x7fffffffffffffff` (`2**63 - 1`, `9223372036854775807`).
+   It affects the following instructions:
    - `TIMESTAMP` (`0x42`),
    - `NUMBER` (`0x43`).
 
 3. *account address*
-   is a range between `0` and `1461501637330902918203684832716283019655932542975` (`2**160 - 1`).
-   It affects the following opcodes:
+   is a range between `0` and `0xffffffffffffffffffffffffffffffffffffffff` (`2**160 - 1`, `1461501637330902918203684832716283019655932542975`)
+   i.e. the address occupies the 160 low bits of the 256-bit value and the remaining top 96 bits must be zeros.
+   It affects the following instructions:
    - `ADDRESS` (`0x30`),
    - `ORIGIN` (`0x32`),
    - `CALLER` (`0x33`),
@@ -50,8 +51,8 @@ They restrict the results (i.e. values pushed to the stack) of the opcodes liste
    - `CREATE2` (`0xf5`).
 
 4. *buffer size*
-   is a range between `0` and `4294967295` (`2**32 - 1`).
-   It affects the following opcodes:
+   is a range between `0` and `0xffffffff` (`2**32 - 1`, `4294967295`).
+   It affects the following instructions:
    - `CALLDATASIZE` (`0x36`),
    - `CODESIZE` (`0x38`),
    - `EXTCODESIZE` (`0x3b`),