Move non-normative parts of "A" extensions (atomics) into NOTE block (riscv#1822)

* a-st-ext: mark examples as a note

Examples are usually in note blocks already.  Make these consistent.

The text about compare-and-swap example should probably be improved and
moved above the example.

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

* 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]>

---------

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

Author: radimkrcmar

src/a-st-ext.adoc

@@ -227,8 +227,8 @@ instruction unless the _rl_ bit is also set. LR._rl_ and SC._aq_
 instructions are not guaranteed to provide any stronger ordering than
 those with both bits clear, but may result in lower performance.
 
-<<<
-
+[NOTE]
+====
 [[cas]]
 [source,asm]
 .Sample code for compare-and-swap function using LR/SC.
@@ -250,6 +250,7 @@ those with both bits clear, but may result in lower performance.
 LR/SC can be used to construct lock-free data structures. An example
 using LR/SC to implement a compare-and-swap function is shown in
 <<cas>>. If inlined, compare-and-swap functionality need only take four instructions.
+====
 
 [[sec:lrscseq]]
 === Eventual Success of Store-Conditional Instructions
@@ -434,15 +435,15 @@ both imply additional unnecessary ordering as compared to AMOs with the
 corresponding _aq_ or _rl_ bit set.
 ====
 
+[NOTE]
+====
 An example code sequence for a critical section guarded by a
 test-and-test-and-set spinlock is shown in
 Example <<critical>>. Note the first AMO is marked _aq_ to
 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.
@@ -457,9 +458,7 @@ relinquishment.
         # ...
         amoswap.w.rl x0, x0, (a0) # Release lock by storing 0.
 
-[NOTE]
-====
-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]
 ====