Improve svn copy documentation

[romain]

Summary

The documentation incorrectly states that

% svn copy foo.c bar.c

and

% cp foo.c bar.c
% svn add bar.c

are equivalent. However, the first one keeps foo.c history and the second gets
rid of it, which is the reason why we do repocopies.

Fix this and explain the difference.

[romain]

Added @wblock as a reviewer because he did the commit that introduced this ambiguity :-)

[bcr]

Typo fix.

en_US.ISO8859-1/articles/committers-guide/article.xml

968 ↗

(On Diff #34000)

s/usualy/usually/

[romain]

Fix typo

[romain]

Typo fixed

[allanjude]

Approved

[wblock]

head/en_US.ISO8859-1/articles/committers-guide/article.xml

968

This sentence long and has a lot of information. Better to split.

Also, best to avoid possessives, which are rarely needed.

Suggested:

Using <command>svn copy</command> is usually preferable because it
keeps the history of the original file with the new file.  When the original file is manually
copied to a new file, then added to version control, the new file does not have the
commit history of the original.