ZOOKEEPER-4391: Allow custom JVM heap sizing - #2460

Nice and simple concept, but I think the word "auto" is the wrong word to use. It appears to be chosen to manually configure JVM flags that affect automatic heap sizing. However, what it is actually doing is removing any automatically placed heap configuration fields. You can use that to add flags manually that do automatic heap sizing, but you can also use that to just omit those heap options for any other reason. With the word "auto", users may be confused that you need to set it to remove an automatic addition of heap options, so that you can use it (optionally) for different heap options.

So, I think something along the lines of "none", "skip", "custom", or "omit" should be used instead.

The instruction in the documention would then look something like:

Use `manual` to skip adding any heap options to the JVM environment automatically, so you can manually set any heap related flags yourself. For example:


```bash
ZK_SERVER_HEAP=omit \
SERVER_JVMFLAGS="-XX:InitialRAMPercentage=25 -XX:MaxRAMPercentage=75" \
./zkServer.sh start
```

or

```bash
ZK_SERVER_HEAP=omit \
SERVER_JVMFLAGS="-Xmx2g" \
./zkServer.sh start
```

Great!

I think the word "auto" is the wrong word to use.

So, I think something along the lines of "none", "skip", "custom", or "omit" should be used instead.

Yes, auto was a confusing choice by me. I've updated to use the word omit and updated the docs as you suggested.

"ignore" could also be a candidate. I have no preference, but I think we should phrase the doc sentence carefully so we don't mislead users.

Looks good from my side.