grep docs: describe --no-index further and improve formatting a bit

Improve the description of --no-index, to make it more clear to the users
what this option actually does under the hood, and what's its purpose.
Describe the dependency between --no-index and either of the --cached and
--untracked options, which cannot be used together.

As part of that, shuffle a couple of the options, to make the documentation
flow a bit better, because it makes more sense to describe first the options
that have something in common, and to after that describe an option that does
something differently.  In more detail, --cached and --untracked both leave
git-grep(1) in the usual state, in which it treats the directory as a local
git repository, unlike --no-index that makes git-grep(1) treat the directory
not as a git repository.

While there, improve the descriptions of grep worker threads a bit, to give
them better context.  Adjust the language a bit, to avoid addressing the
reader directly, which is in general preferred in technical documentation,
because it eliminates the possible element of persuading the user to do
something.  In other words, we should be telling the user what our software
can do, instead of telling the user what to do.

Also perform some minor formatting improvements, to make it clear it's the
git commands, command parameters, and configuration option names.

Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -28,7 +28,7 @@ SYNOPSIS
 	   [-f <file>] [-e] <pattern>
 	   [--and|--or|--not|(|)|-e <pattern>...]
 	   [--recurse-submodules] [--parent-basename <basename>]
-	   [ [--[no-]exclude-standard] [--cached | --no-index | --untracked] | <tree>...]
+	   [ [--[no-]exclude-standard] [--cached | --untracked | --no-index] | <tree>...]
 	   [--] [<pathspec>...]
 
 DESCRIPTION
@@ -45,13 +45,21 @@ OPTIONS
 	Instead of searching tracked files in the working tree, search
 	blobs registered in the index file.
 
---no-index::
-	Search files in the current directory that is not managed by Git.
-
 --untracked::
 	In addition to searching in the tracked files in the working
 	tree, search also in untracked files.
 
+--no-index::
+	Search files in the current directory that is not managed by Git,
+	or by ignoring that the current directory is managed by Git.  This
+	is rather similar to running the regular `grep(1)` utility with its
+	`-r` option specified, but with some additional benefits, such as
+	using pathspec patterns to limit paths;  see the 'pathspec' entry
+	in linkgit:gitglossary[7] for more information.
++
+This option cannot be used together with `--cached` or `--untracked`.
+See also `grep.fallbackToNoIndex` in 'CONFIGURATION' below.
+
 --no-exclude-standard::
 	Also search in ignored files by not honoring the `.gitignore`
 	mechanism. Only useful with `--untracked`.
@@ -248,8 +256,8 @@ providing this option will cause it to die.
 	a non-zero status.
 
 --threads <num>::
-	Number of grep worker threads to use.
-	See `grep.threads` in 'CONFIGURATION' for more information.
+	Number of `grep` worker threads to use.  See 'NOTES ON THREADS'
+	and `grep.threads` in 'CONFIGURATION' for more information.
 
 -f <file>::
 	Read patterns from <file>, one per line.
@@ -336,9 +344,9 @@ The `--threads` option (and the `grep.threads` configuration) will be ignored wh
 `--open-files-in-pager` is used, forcing a single-threaded execution.
 
 When grepping the object store (with `--cached` or giving tree objects), running
-with multiple threads might perform slower than single threaded if `--textconv`
-is given and there are too many text conversions. So if you experience low
-performance in this case, it might be desirable to use `--threads=1`.
+with multiple threads might perform slower than single-threaded if `--textconv`
+is given and there are too many text conversions.  Thus, if low performance is
+experienced in this case, it might be desirable to use `--threads=1`.
 
 CONFIGURATION
 -------------