[Java] Document BOUNDS_CHECKING_ENABLED

[asfimport]

According to @animeshtrivedi, BOUNDS_CHECKING_ENABLED has significant implication on performance.

We should document this better and maybe revisit the default value. 

https://github.com/apache/arrow/blob/master/java/memory/src/main/java/org/apache/arrow/memory/BoundsChecking.java

Reporter: Li Jin / @icexelloss
Assignee: Animesh Trivedi / @animeshtrivedi

PRs and other links:

• GitHub Pull Request #3032

_{Note: This issue was originally created as ARROW-3493. Please see the migration documentation for further details.}

[asfimport]

Animesh Trivedi / @animeshtrivedi:
So, I propose that we

i) rename the variable from "drill.enable_unsafe_memory_access" to "arrow.enable_unsafe_memory_access"

ii) set the default to false. This can be enabled if user explicitly wants to debug and check ranges. 

iii) document this flag (and perhaps the netty one too) in the Java user documentation. 

If we agree then I will open a pull request.

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 3032
#3032