docs: gitdiffcore: -G and -S: Use regex/string placeholders

In the rest of the documentation (and in the code) we use `regex` and
`string` as `-G` and `-S` argument placeholders.  While
`regular-expression` and `block-of-text` are a bit easier to read, it is
a bit consistent.

And we could assume that everyone who uses git should be able to
understand that a "string" and a "block-of-text", as well as a "regex"
and "regular-expression" are the same thing.  So, using a shorter
version is also more consistent.

Signed-off-by: Junio C Hamano <[email protected]>

Author: ilya-bobyr

Documentation/gitdiffcore.txt

@@ -245,26 +245,25 @@ diffcore-pickaxe: For Detecting Addition/Deletion of Specified String
 
 This transformation limits the set of filepairs to those that change
 specified strings between the preimage and the postimage in a certain
-way.  -S<block-of-text> and -G<regular-expression> options are used to
-specify different ways these strings are sought.
+way.  `-S<string>` and `-G<regex>` options are used to specify
+different ways these strings are sought.
 
-"-S<block-of-text>" detects filepairs whose preimage and postimage
-have different number of occurrences of the specified block of text.
+`-S<string>` detects filepairs whose preimage and postimage
+have different number of occurrences of the specified _<string>_.
 By definition, it will not detect in-file moves.  Also, when a
 changeset moves a file wholesale without affecting the interesting
 string, diffcore-rename kicks in as usual, and `-S` omits the filepair
 (since the number of occurrences of that string didn't change in that
 rename-detected filepair).  When used with `--pickaxe-regex`, treat
-the <block-of-text> as an extended POSIX regular expression to match,
+the _<string>_ as an extended POSIX regular expression to match,
 instead of a literal string.
 
-"-G<regular-expression>" (mnemonic: grep) detects filepairs whose
-textual diff has an added or a deleted line that matches the given
-regular expression.  This means that it will detect in-file (or what
-rename-detection considers the same file) moves, which is noise.  The
-implementation runs diff twice and greps, and this can be quite
-expensive.  To speed things up, binary files without textconv filters
-will be ignored.
+`-G<regex>` (mnemonic: grep) detects filepairs whose textual diff has
+an added or a deleted line that matches the given _<regex>_.  This
+means that it will detect in-file (or what rename-detection considers
+the same file) moves, which is noise.  The implementation runs diff
+twice and greps, and this can be quite expensive.  To speed things up,
+binary files without textconv filters will be ignored.
 
 When `-S` or `-G` are used without `--pickaxe-all`, only filepairs
 that match their respective criterion are kept in the output.  When