[Test] added trace logging to refresh action to note which cluster version was used

Increase logging level for SearchWithRandomExceptionsTests.testRandomExceptions & MinDocCountTests to use the above and log cluster states on changes.
[TEST] Wait for nodes to join before asserting on cluster stats
Show stacktrace of startup exception

Whether or not the stacktrace is displayed is controlled by bootstrap
log level setting, so that bootstrap: DEBUG displays the stack trace on
output, like it does on log

Closes #5102
[DOCS] Fixed typo
Adding javadoc to UpdateRequestBuilder for a couple of details it took me a while to find.
[DOCS] A few grammar and word use corrections
log full exception in InternalClusterInfoService unless it's a ClusterBlockException
Add `getAsRatio` to Settings class, allow DiskThresholdDecider to take percentages

Adds new RatioValue class that parses ratios between 0-100% expressed in
either floating-point (0.13) or percentage (51.12%) notation.

Closes #5690