[SPARK-3443][MLLIB] update default values of tree:

[mengxr]

Adjust the default values of decision tree, based on the memory requirement discussed in #2125 :

1. maxMemoryInMB: 128 -> 256
2. maxBins: 100 -> 32
3. maxDepth: 4 -> 5 (in some example code)

@jkbradley

[mengxr]

Jenkins, test this please.

[jkbradley]

@mengxr I could only find a few items missed (in comments): Look in DecisionTree for "(suggested value:"

[mengxr]

test this please

[SparkQA]

QA tests have started for PR 2322 at commit 5900445.

• This patch merges cleanly.

[jkbradley]

@mengxr LGTM

[SparkQA]

QA tests have finished for PR 2322 at commit 5900445.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • logDebug("isMulticlass = " + metadata.isMulticlass)
  • logDebug("isMulticlass = " + metadata.isMulticlass)

[SparkQA]

QA tests have started for PR 2322 at commit cda453a.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2322 at commit cda453a.

• This patch fails unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

test this please

[SparkQA]

QA tests have started for PR 2322 at commit cda453a.

• This patch merges cleanly.

[SparkQA]

QA tests have finished for PR 2322 at commit cda453a.

• This patch passes unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[mengxr]

I've merged this into master. Thanks @jkbradley for review!