There are very well documented uCs. I have to admit , is not like in the good old days, but there are decent documented uCs.

One thing is happening: as uC (and ASIcs) are getting more complex and complicated, more features are added and not fully (or at all) documented.

Atmel has shockingly good documentation for their AVR CPUs. The datasheets are always thorough, usually include protocol timing diagrams, psuedocode examples for using peripherals, lots and lots of vital information. Their datasheets also link out to additional documentation like their instruction set manual. Now that is a good read.

Love Atmel. They're not perfect, but they're very good and they try pretty hard.

TI deleted all the I2C register programming information (40 pages!) from the docs for a widget I was using and it took them months to put it back. The device was literally unusable in any form without that information. Insane.