doc: increase Buffer.concat() documentation

This commit adds documentation for two edge cases in Buffer.concat(). Those cases are:

• totalLength is specified, but is not an integer.
• The combined buffer length is greater than totalLength.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

Fixes: https://github.com/nodejs/node/issues/11605