Add a note to CONTRIBUTING.md explaining the required inter-line char…

…acter alignment of multiline method signatures in doc/stdlib/*.rst.

CONTRIBUTING.md

@@ -104,6 +104,21 @@ If you want to edit an existing docstring signature, you **first** have to chang
 edit the helpdb.jl or inline method docstrings. The existing signatures in the `doc/stdlib/*.rst` files are pattern matched to base docstrings and the new content overwrites the content in `doc/stdlib/`.
 The signature definitions **must** be in sync or else the pattern match will fail and documentation will be lost in the result.
 To add entirely new methods to the `stdlib` documentation, first add the signature in the appropriate `doc/stdlib/*.rst` file before writing the docstring, rebuilding Julia, and re-running `doc/genstdlib.jl`.
+Pattern matching requires that multiline method signatures' inter-line character alignment in `doc/stdlib/*.rst` match that in the corresponding docstring. For example, docstring
+```julia
+"""
+ foo(bar, baz,
+ qux, quux)
+
+Foo `bar`, `baz`, `qux`, and `quux`.
+"""
+```
+will only match entries in `doc/stdlib/*.rst` of the form
+```rst
+.. function:: foo(bar, baz,
+.............. qux, quux)
+```
+where `.` in the preceding snippet's second line is wildcard, and their number preceding indent-` qux, quux)` matches the character count of `.. function:: `.
 
 It is encouraged to write all new docstrings in Markdown markup. If you need to write a more complicated docstring that contains cross-references or citations it can be written in a restructured text codeblock.
 Many of the existing docstrings are currently restructured text codeblocks and these will be transitioned to Markdown over time. RST codeblocks are delineated with the triple-quote (\`\`\`rst \`\`\`) Makdown codeblock syntax.