Delete a comment describing a design that was removed, and check the rest

Two comments in the native assembler survived the changes that made them
false, and both are the kind that misleads rather than merely ages.

Asm.asm still explained an eighteen kilobyte buffer that the whole output
had to fit in "because a file is written in one call and there is nowhere
to put half of one" - which stopped being true when the assembler learned
to stream, and the variable it described, ImgRoom, does not exist any more.
It was sitting in front of the comment that replaced it, so the paragraph a
reader met first described the design that had been taken out. Replaced by
what is actually there: no limit but the disk, one block at a time through
a window in scratch.

scratch.asm said the system keeps below 0x1000 twelve lines above the
paragraph explaining that the system's half of Data Memory was doubled to
0x1FFF. A stale number next to its own correction is worse than a stale
number alone, because which one a reader believes is a coin toss.

docs.sh now checks both of the facts scratch.asm asserts about the machine
around it: the floor it claims the system keeps below, against the CosmOS
README, and the address its buffers start at, against where the assembler's
own data actually ends. Neither is enforced by a line of code anywhere -
the map is a comment, deliberately, because reserving the buffers would put
22K of zeroes in the file and the assembler could not load itself - so a
check is the only thing that can hold them. Both fail when broken; the
second reports the exact overlap.

Programs/CosmOS/Assembler/scratch.asm

@@ -12,9 +12,14 @@
 ;
 ; None of this is initialised data. It is scratch, wanted only while the assembler is
 ; running, and while it is running everything above its own data is free: the system keeps
-; below 0x1000, the staging area is only in use during a load, and the Stack comes down
+; below 0x1FFF, the staging area is only in use during a load, and the Stack comes down
 ; from the top. So the addresses are written down here and the file carries none of it.
 ;
+; That sentence said 0x1000 for a while after the system's half of Data Memory was
+; doubled, twelve lines above the paragraph that explains the doubling. A stale number is
+; bad enough; a stale number sitting next to the correction is worse, because whichever
+; one a reader takes is a coin toss.
+;
 ;   0x4000  6144   the label index, 1536 entries of four
 ;   0x5800 16384   the label names, packed end to end
 ;   0x9800   256   one block of the output file, on its way to the disk
@@ -30,9 +35,12 @@
 ; and the map stayed where it was, leaving sixteen kilobytes between the two that nothing
 ; touched.
 ;
-; Starting at 0x4000 takes that back. The assembler's data is 4,114 bytes from 0x2000, so
-; there is still nearly four kilobytes of slack in front of this - and room for its data to
-; double before the two would meet.
+; Starting at 0x4000 takes that back. The assembler's data is a little over four kilobytes
+; from 0x2000, so there is still nearly four kilobytes of slack in front of this - and room
+; for its data to double before the two would meet. `make test` measures that gap now
+; rather than trusting this paragraph, and measures the floor above as well, because both
+; of those numbers describe the machine AROUND this file and neither is enforced by a line
+; of code anywhere.
 ;
 ; THE ROOM WENT TO ALL THREE OF THE BUFFERS THAT WERE FULL, and there turned out to be
 ; three rather than one. The output was the obvious wall - cosmos.bin was 13,245 bytes