docs: discuss caching personal access tokens

Describe problems storing personal access tokens in git-credential-cache
and suggest alternatives.

Research suggests that many users are confused about this:

> the point of passwords is that (ideally) you memorise them [so]
> they're never stored anywhere in plain text. Yet GitHub's personal
> access token system seems to basically force you to store the token in
> plain text?

https://stackoverflow.com/questions/46645843/where-to-store-my-git-personal-access-token#comment89963004_46645843

Documentation/git-credential-cache.txt

@@ -78,6 +78,23 @@ variable (this example increases the cache time to 1 hour):
 $ git config credential.helper 'cache --timeout=3600'
 -------------------------------------------------------
 
+PERSONAL ACCESS TOKENS
+----------------------
+
+Some remotes accept personal access tokens, which are randomly
+generated and hard to memorise. They typically have a lifetime of weeks
+or months.
+
+git-credential-cache is inherently unsuitable for persistent storage of
+personal access tokens. The credential will be forgotten after the cache
+timeout. Even if you configure a long timeout, credentials will be
+forgotten if the daemon dies.
+
+To avoid frequently regenerating personal access tokens, configure a
+credential helper with persistent storage. Alternatively, configure an
+OAuth credential helper to generate credentials automatically. See
+linkgit:gitcredentials[7].
+
 GIT
 ---
 Part of the linkgit:git[1] suite