Issue #6042: lnotab-based tracing is very complicated and isn't documented very well. There were at least 3 comment blocks purporting to document co_lnotab, and none did a very good job. This patch unifies them into Objects/lnotab_notes.txt which tries to completely capture the current state of affairs. I also discovered that we've attached 2 layers of patches to the basic tracing scheme. The first layer avoids jumping to instructions that don't start a line, to avoid problems in if statements and while loops. The second layer discovered that jumps backward do need to trace at instructions that don't start a line, so it added extra lnotab entries for 'while' and 'for' loops, and added a special case for backward jumps within the same line. I replaced these patches by just treating forward and backward jumps differently.