a-st-ext: remove manual page breaks

We should improve the tooling instead of adding manual page breaks.

Also replace "above" with a reference, because "above" is not correct
anymore and most importantly, it's not acceptable for a spec.

This does not imply any acceptability of the the rest of the sentence,
I just didn't want to leave the spec in a worse shape.

Signed-off-by: Radim Krčmář <[email protected]>

src/a-st-ext.adoc

@@ -230,8 +230,6 @@ instructions are not guaranteed to provide any stronger ordering than
 those with both bits clear, but may result in lower performance.
 ====
 
-<<<
-
 [NOTE]
 ====
 [[cas]]
@@ -452,8 +450,6 @@ order the lock acquisition before the critical section, and the second
 AMO is marked _rl_ to order the critical section before the lock
 relinquishment.
 
-<<<
-
 [[critical]]
 [source,asm]
 .Sample code for mutual exclusion. `a0` contains the address of the lock.
@@ -468,7 +464,7 @@ relinquishment.
         # ...
         amoswap.w.rl x0, x0, (a0) # Release lock by storing 0.
 
-We recommend the use of the AMO Swap idiom shown above for both lock
+We recommend the use of the AMO Swap idiom shown in <<critical>> for both lock
 acquire and release to simplify the implementation of speculative lock
 elision. cite:[Rajwar:2001:SLE]
 ====