Explain that failure of memory wipe is usually no big deal

Originally created by @sajolida on #8156 (Redmine)

We currently mention memory wipe in two places in our documentation:

• https://tails.boum.org/doc/advanced_topics/cold_boot_attacks/
• https://tails.boum.org/support/known_issues/#index20h2

Users are sometimes confused by the notion of memory (RAM vs hard disc) and the notion of trace (which are ephemeral in the case of RAM). So we should make it clearer that: a failure of the memory wipe only mean leaving very ephemeral traces on the device and is no big deal is most cases.