scripts: improve strip-whitespace.py

previous version barfed when it was called on a directory
(which could happen for example when called with a wildcard).
Also, add comment.
Typo fix: \CadenzaOn/Off -> \cadenzaOn/Off
Doc: Change Balloon Text example

Issue 3608

Remove redundant \balloonLengthOff command

Change paragraph to say 'does not' rather, than currently stated, as 'does'
NR: Correct explanation of ledger line override

Issue 3556

Correct the explanation of how the ledger-line-thickness
override is calculated.
Issue 3611: Parser: remove non-closed expressions from function_arglist_nonbackup_common
Revert "Issue 3205: opening bar check causes crash if \score contains the \midi block"

This reverts commit c83a3afbc51c62769cbb8963371d37b312bbcd42.

The replacement of Simple_music_iterator with Music_iterator in
lily/bar-check-iterator.cc made bar checks at the start of music
expressions inoperative altogether, even when not at a bar boundary:

{ a a { | } a a }

stopped producing a diagnostic.  Issue 3205 instead needs to be fixed
together with issue 2392 which apparently may be called "Segfault in
Midi when Simple_music_iterator is used at the beginning of an
implicitly created context".
Issue 3603: Make \tweak ... Grob equivalent to \override rather than \once \override

This simplifies several uses of tweak for both tweak and override.
When desired, one can use \once \tweak in order to get either tweak or
override-once semantics: in the case of the tweak semantics, \once
will just ignore the contained expression.