Help documentation for source-subdir is out of date

[Kyle Fazzari]

`snapcraft help sources` shows the following (snipped for clarity):

    source-subdir: path

    Snapcraft will checkout the repository or unpack the archive referred
    to by the 'source' keyword into parts/<part-name>/src/ but it will only
    copy the specified subdirectory into parts/<part-name>/build/

That's not actually true nowadays. It copies the entire parts/<part-name>/src/ into parts/<part-name>/build/ but then sets the plugin's builddir to parts/<part-name>/build/<subdir>. This was changed to fix #1549676, but the documentation was never updated.

[Leo Arias (elopio)]

Setting priority to high because the inaccurate docs have been there for a long time.

[Kyle Fazzari (kyrofa)]

Fix: https://github.com/snapcore/snapcraft/pull/1177