HPA Algorithm Information Improvements (#9780)

* Update HPA docs with more algorithm details

The HPA docs pointed to an out-of-date document for information on the
algorithm details, which users were finding confusing.  This sticks a
section on the algorithm in the HPA docs instead, documenting both
general behavior and corner cases.

* Add glossary info, HPA docs on quantities

People often ask about the quantity notation when working with the
metrics APIs, so this adds a glossary entry on quantities (since they're
used elsewhere in the system), and a short explantation in the HPA walkthough.

* Information about HPA readiness and stabilization

This adds information about the new changes to HPA readiness and
stabilization from kubernetes/features#591, and other minor changes that
landed in Kubernetes 1.12.

* Update horizontal-pod-autoscale.md

content/en/docs/reference/glossary/quantity.md

@@ -0,0 +1,30 @@
+---
+title: Quantity
+id: quantity
+date: 2018-08-07
+full_link:
+short_description: >
+  A whole-number representation of small or large numbers using SI suffixes.
+
+aka: 
+tags:
+---
+ A whole-number representation of small or large numbers using SI suffixes.
+
+<!--more--> 
+
+Quantities are representations of small or large numbers using a compact,
+whole-number notation with SI suffixes.  Fractional numbers are represented
+using milli-units, while large numbers can be represented using kilo-units,
+mega-units, giga-units, etc.
+
+For instance, the number `1.5` is represented `1500m`, while the number `1000`
+can be represented as `1k`, and `1000000` as `1M`. You can also specify
+binary-notation suffixes; the number 2048 can be written as `2Ki`.
+
+The accepted decimal (power-of-10) units are `m` (milli), `k` (kilo,
+intentionally lowercase), `M` (mega), `G` (giga), `T` (terra), `P` (peta),
+`E` (exa).
+
+The accepted binary (power-of-2) units are `Ki` (kibi), `Mi` (mebi), `Gi` (gibi),
+`Ti` (tebi), `Pi` (pebi), `Ei` (exbi).