Fixed documentation error with 'Press Key' keyword

Escaped double baskslash within keyword documentation. Apparently RF escapes
characters so when it sees the double backslash is interprets this as a single
backslah when indeed it should be read as a double backslash.
This fixes issues #200.
Clarified documentation of XPath related keywords.

Added note to both 'Xpath Should Match X Times' and 'Get Matching Xpath Count'
keywords stating that it is incorrect to add the 'xpath=' prefix to the
locator as this is already assumed by the keyword.
This fixes issue #228.
Merge branch 'doc-press-key' into documentation-release
Merge branch 'documentation-release' into doc-xpath
Merge branch 'doc-xpath' into documentation-release
Merge branch 'documentation-release' into doc-debug