Document NODE_MANY_ACCEPTS environment variable.

While investigating a problem with electron/dat I found the NODE_MANY_ACCEPTS environment variable exclusively used in the win32 context.

node/lib/net.js

Lines 1757 to 1760 in 02fef8a

if (simultaneousAccepts === undefined) {
simultaneousAccepts = (process.env.NODE_MANY_ACCEPTS &&
process.env.NODE_MANY_ACCEPTS !== '0');
}

However I was not able to find any documentation of why this is necessary or what it does in the environment variable doc.