gdb/help: Update help message for target record-core

gdb/help: Update help message for target record-core

Before this commit, the help message for target record-core is the same
as the help message for target record-full, which is the following:
    Log program while executing and replay execution from log.

For one, having the same message is unhelpful, since it doesn't tell
users what the difference between the two is.  But, more importantly,
that message seems to also be incorrect, since attempting to execute the
inferior forward from a restored file past the end of history will crash
GDB, since there isn't an actual live inferior to run.

To fix this, the help text is updated to the following:
    Load a saved execution log, allowing replay of the last instructions
This message doesn't imply that future execution is supported, while it
shows that replaying within recorded instructions *is* supported.

Approved-By: Tom Tromey <tom@tromey.com>