Fix some types and wordings (#582)

### Before requesting review
- [x] I have reviewed the code myself

src/fuel-vm/index.md

@@ -183,7 +183,7 @@ A call frame consists of the following, word-aligned:
 
 ## Access rights
 
-Only memory that has been allocated is accessible. Attempting to read or write memory that has not been allocated will result in VM panic. Similarly reads or writes that cross from the stack to the heap will panic. Note stack remains readable even after stack frame has been shrunk. However, if heap is alter expanded to cover that area, the crossing read prohibition still remains. In other word, memory between highest-ever `$sp` value and current `$hp` is inaccessible.
+Only memory that has been allocated is accessible. Attempting to read or write memory that has not been allocated will result in VM panic. Similarly reads or writes that cross from the stack to the heap will panic. Note that stack remains readable even after stack frame has been shrunk. However, if the heap is afterwards expanded to cover that area, the crossing read prohibition still remains. In other word, memory between highest-ever `$sp` value and current `$hp` is inaccessible.
 
 ### Ownership
 
@@ -201,4 +201,4 @@ If the context is internal, the owned memory range for a call frame is:
 
 ### Executablity
 
-Memory is only executable in range `[$is, $ssp)`. Attempting to execute instructions outside these boundaries will panic. This area never overlaps with writable registers, essentially providing [W^X](https://en.wikipedia.org/wiki/W%5EX) protection.
+Memory is only executable in range `[$is, $ssp)`. Attempting to execute instructions outside these boundaries will cause a panic. This area never overlaps with writable memory, essentially providing [W^X](https://en.wikipedia.org/wiki/W%5EX) protection.

src/fuel-vm/instruction-set.md

@@ -1835,7 +1835,7 @@ Panic if:
 
 Increment `$fp->codesize`, `$ssp` by `$rC` padded to word alignment. Then set `$sp` to `$ssp`.
 
-This instruction can be used to concatenate the code of multiple contracts together. It can only be used when the stack area of the call frame is unused (i.e. prior to being used).
+This instruction can be used to concatenate the code of multiple contracts together. It can only be used when the stack area of the call frame is zero-sized.
 
 ### `LOG`: Log event