Instead of using the home_bump for Z to move close to the bed before doing a slow probe, use the probe travel height.

If `PROBE_DOUBLE_TOUCH` wasn't enabled, the probe move could be way too short! See #4701 for example.

Add note that Viki 2.0 wiring diagram v1.2 has a mistake in it;

These '{' are important to avoid the redefinition of `unsigned char _sreg = SREG;`
at the same level.
Used in
```
#define CRITICAL_SECTION_START  unsigned char _sreg = SREG; cli();
```

Follow-up to #4666

Also improve the documentation of this and related options, and set
EXTRUDE_MINTEMP to 170 everywhere.