[docs][clang] refactor diagnostic spec instruction and grammar nit (#213432)
A grammar nit found in the description of the Clang Diagnostic Subsystem
is improved.
Replace the instruction mentioning the use of "slash-newline" for
writing multiple diagnostics with the appropriate "backslashes" as that's
what is actually used for separting multiple diagnostics into more than
one line.
The change makes it more obvious on how to implement multi-line
diagnostics by dropping the slightly ambigious slash-newline.
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Signed-off-by: Hardik Kumar <hardikxk@gmail.com>