docs: make the documentation for --etag-save match the program behaviour

docs: make the documentation for --etag-save match the program behaviour

When using curl with the option `--etag-save` I expected it to save the
ETag without its surrounding quotes, as stated by the documentation in
the repository and by the generated man pages.

My first endeavour was to fix the program, but while investigating the
history of the relevant parts, I discovered that curl once saved the
ETag without the quotes.  This was undone by Daniel Stenberg in commit
`98c94596f5928840177b6bd3c7b0f0dd03a431af`, therefore I decided that in
this case the documentation should be adjusted to match the behaviour of
curl.

The changed save behaviour also made parts of the `--etag-compare`
documentation wrong or superfluous, so I adjusted those accordingly.

Closes #7429